If I understand the sequence correctly, you initially backed up your iPhone 15 Pro Max to iCloud, and the backup was nearly 240 GB. You then erased the iPhone 15 Pro Max completely, although it is unclear why the device was wiped.
Later, you restored an iPhone X using an iCloud Backup. This part of the sequence raises an important question because an iCloud Backup created from a newer iOS version generally cannot be restored to a device running an older, incompatible iOS version. The iPhone X would need to be running an iOS version that supports restoring that backup. Therefore, it is important to establish exactly which iOS versions were running on both devices and which backup was actually used.
You now have a new iPhone 15 Pro, but the earlier iCloud Backup is apparently no longer appearing as an available backup.
There are two important possibilities here:
1. The iPhone 15 Pro Max was erased after the backup, and the subsequent data state affected what was stored in iCloud
This needs to be separated into iCloud Backup and iCloud Sync, because they are not the same thing.
An iCloud Backup is a snapshot of data from the device at the time the backup was created. It does not continuously mirror every change made on the iPhone.
However, services such as iCloud Photos, iCloud Drive, Contacts, Notes, Messages in iCloud, etc. are synchronisation services. When data is synced through iCloud, deleting or changing the data on one device can cause the same change to propagate to iCloud and subsequently to other devices using the same Apple Account.
Therefore, if you deleted data from the iPhone after the original backup, that does not automatically mean the old iCloud Backup was changed. But if you subsequently created another iCloud Backup after the device had been erased or its data had been removed, that newer backup could replace the previous backup for that device.
In simple terms:
Original iPhone 15 Pro Max → 240 GB backup → iPhone subsequently erased/changed → another backup performed → newer backup may replace the previous backup.
If the later backup was made when the iPhone contained little or none of the original data, the newer backup could therefore be much smaller and would no longer contain the information that was present in the earlier backup.
2. The fact that the backup was restored to the iPhone X needs further clarification
This is the most questionable part of the sequence.
A backup made from an iPhone 15 Pro Max cannot simply be restored to an iPhone X if the iPhone X is running an iOS version older than the version required by the backup. Apple generally requires the destination device to be running a compatible iOS version.
Therefore, if the iPhone X was successfully restored from that backup, we need to determine:
- Which iOS version was running on the iPhone 15 Pro Max when the backup was created?
- Which iOS version was running on the iPhone X?
- Was the iPhone X updated before the restoration?
- Was the 240 GB backup actually selected, or was another, older backup selected?
- Was the original 240 GB backup still present when the iPhone X was restored?
- Was another backup subsequently created from the iPhone X?
These details could explain why the original backup is no longer available.
One more important point: the Apple Account
iCloud Backup is associated with the Apple Account that created it.
If your new iPhone 15 Pro is signed in with a different Apple Account from the one used on the iPhone 15 Pro Max, the old backup will not normally appear as an available backup during restoration.
So first verify that the new iPhone 15 Pro is signed in with the exact same Apple Account that was used on the iPhone 15 Pro Max when the 240 GB backup was created.
If it is the same account and the backup still does not appear, then the next question is whether the original backup was replaced, deleted, expired, or otherwise no longer available.
In short
The most important thing is not to keep erasing, restoring, or creating new backups on the devices until the situation is understood. A new backup can potentially replace an older backup for the same device, while synchronised iCloud data follows a different mechanism altogether.
Also, don't confuse "data disappeared from the iPhone" with "the iCloud Backup was deleted." Those are two different events, and determining which one occurred is critical to recovering the original data.
Good luck — and hopefully that 240 GB backup is still hiding somewhere rather than having taken an unscheduled retirement!