Pretty weird issue. Tried to allocate more space to my win 10 VM by changing the number. Wanted a bit more and is now stuck on 13T, which is a bit steep. No matter what I try, it falls back to 13T when I hit enter

I'm guessing the 13TB was successfully applied at some point, and it's programmed to not allow reductions in size for data loss reasons. If you reduce the size of a vdisk, it's entirely possible to corrupt it if the space reduction discards a used portion of the image. I suspect you will need to manually reduce the size using the command line, or back up the VM using windows image backup or some other partition aware backup like acronis, create a new VM with the size of disk you want and restore your backup.