Bootcamp partition space disappeared after Bootcamp Assistant removal failed

Sure thing, here goes:

tigro@MacBook-Air ~ % diskutil apfs list

APFS Container (1 found)
|
+-- Container disk1 C02F9F4E-46B4-4E4F-8004-169895879D08
    ====================================================
    APFS Container Reference:     disk1
    Size (Capacity Ceiling):      200790749184 B (200.8 GB)
    Capacity In Use By Volumes:   159519363072 B (159.5 GB) (79.4% used)
    Capacity Not Allocated:       41271386112 B (41.3 GB) (20.6% free)
    |
    +-< Physical Store disk0s2 8A1262CF-40B9-4FE7-8EA8-97885D518F3C
    |   -----------------------------------------------------------
    |   APFS Physical Store Disk:   disk0s2
    |   Size:                       250790436864 B (250.8 GB)
    |
    +-> Volume disk1s1 9060519F-8CE4-3240-A7AF-9D2EB92F73CE
    |   ---------------------------------------------------
    |   APFS Volume Disk (Role):   disk1s1 (Data)
    |   Name:                      Macintosh HD – dane (Case-insensitive)
    |   Mount Point:               /System/Volumes/Data
    |   Capacity Consumed:         139794690048 B (139.8 GB)
    |   Sealed:                    No
    |   FileVault:                 Yes (Unlocked)
    |
    +-> Volume disk1s2 0E2C2B5E-0C5E-4E80-B48F-20D934BBA8B9
    |   ---------------------------------------------------
    |   APFS Volume Disk (Role):   disk1s2 (Preboot)
    |   Name:                      Preboot (Case-insensitive)
    |   Mount Point:               /System/Volumes/Preboot
    |   Capacity Consumed:         389234688 B (389.2 MB)
    |   Sealed:                    No
    |   FileVault:                 No
    |
    +-> Volume disk1s3 85F0821D-53AB-42D9-BA07-AF72EC58AA99
    |   ---------------------------------------------------
    |   APFS Volume Disk (Role):   disk1s3 (Recovery)
    |   Name:                      Recovery (Case-insensitive)
    |   Mount Point:               Not Mounted
    |   Capacity Consumed:         622211072 B (622.2 MB)
    |   Sealed:                    No
    |   FileVault:                 No
    |
    +-> Volume disk1s4 B60D2F3C-94D0-4258-A5EB-0A7C92E51C03
    |   ---------------------------------------------------
    |   APFS Volume Disk (Role):   disk1s4 (VM)
    |   Name:                      VM (Case-insensitive)
    |   Mount Point:               /System/Volumes/VM
    |   Capacity Consumed:         3222298624 B (3.2 GB)
    |   Sealed:                    No
    |   FileVault:                 No
    |
    +-> Volume disk1s5 29671E89-9A39-4284-9035-7987D7DC3272
        ---------------------------------------------------
        APFS Volume Disk (Role):   disk1s5 (System)
        Name:                      Macintosh HD (Case-insensitive)
        Mount Point:               /Volumes/Macintosh HD 1
        Capacity Consumed:         15355846656 B (15.4 GB)
        Sealed:                    Broken
        FileVault:                 Yes (Unlocked)
        Encrypted:                 No
        |
        Snapshot:                  A7F7CFDE-537A-4473-A237-B01C0146C2B8
        Snapshot Disk:             disk1s5s1
        Snapshot Mount Point:      /
        Snapshot Sealed:           Yes

and

tigro@MacBook-Air ~ % diskutil apfs resizeContainer disk0s2 limits

Resize limits for APFS Physical Store partition disk0s2:
  Current Physical Store partition size on map:   250.8 GB (250790436864 Bytes)
  Minimum (constrained by file/snapshot usage):   163.6 GB (163644964864 Bytes)
  Recommended minimum (if used with macOS):       174.4 GB (174382383104 Bytes)
  Maximum (constrained by partition map space):   250.8 GB (250790436864 Bytes)

Nope, I don't have a Time Machine backup :(

Haven't tried booting into recovery. Should I? What should I do when I get there?

Yep, I was able to choose 87GB - however as you mentioned, it results in an error :( Here's the log:

Dzielę dysk „APPLE SSD SD0256F Media” (disk0) na partycje {{dividing up the disk into partitions}}

Podczas zmiany wielkości woluminu startowego komputer może przestać odpowiadać. {{during the procedure the computer may become unresponsive}

Wykonuję operację 1 z 2: Dodaj „Bez nazwy” 87,04 GB, zmniejszając kontener disk1 „Macintosh HD” (disk0s2) z 250,79 GB do 163,75 GB… {{pending operation 1 of 2: add "untitled" 87GB, resizing the disk 1 "Macintosh HD" (disk0s2) container from 250GB to 163GB}}
Aligning shrink delta to 87,044,808,704 bytes and targeting a new physical store size of 163,745,628,160 bytes
Determined the minimum size for the targeted physical store of this APFS Container to be 163,745,628,160 bytes
There is not enough free space in this APFS Container for this operation. : (-69605)

Operacja nie powiodła się… {{operation failed}}

(added translations of the parts which were in my system language in curly brackets where needed)

Ha, solved! :D I wanted to try creating a smaller one myself but couldn't find a way as the size text box is greyed out when you choose FAT or exFAT - but the little circle dial on the left hand side still works! After trying out a 40GB partition, an error occurred again, this time later in the process (no error code, just "internal error occurred, try again later") but a new 40GB partition still appeared in disk utility (albeit an exFAT one instead of the FAT we choose in the beginning). So I tried removing it through the same partitioning dialog I created it with in disk utility and then somehow magically my main macOS partition started having full 90GB of free space instead of the 40GB it had before with 50GB floating in the abyss. So all's good now, thanks so much, never would have figured that out on my own!

Yep, I get:

Started APFS operation
Error: -69743: The new size must be different than the existing size

Ah, I see - that's why even though the created partition was smaller than the bootcamp one, it still gave me back all my space upon removal. Thanks for explaining that!