There are two 'modes' of Windows installation. BIOS (legacy) and UEFI. The BIOS mode installation requires the partitions to mimic a MBR disk using a Hybrid MBR. If you change partitions, the MBR and the underlying partitions are out-of-sync. This causes boot issues. Creating a partition is not recommended for such installations. 2014 and older models support a MBR for BIOS mode.
From macOS terminal, post the output of
diskutil list
sudo gpt -vv -r show /dev/disk0
sudo fdisk /dev/disk0
You may temporarily need to disable SIP using csrutil in local recovery.