Happy New Year!
You're absolutely right, you should not have to recreate your monsters in the Bestiary! I suspect this is a known issue and I can offer two potential solutions:
1) work around: move your Bestiary (and any images) to a new location on your harddrive, then go to the options menu and select the new Bestiary location. I recommend to do this without having any campaign open to avoid damaging the campaign monster links.
2) message me on discord, I can share a link to the engineering build where the bug has hopefully been fixed. It would be great if you could try out that. we version and give feedback whether it does indeed fix the issue.