There have been several issues that have occurred subsequent to the Cataina upgrade and iMovie update to 10.1.13. Hard to tell if your issue is related to that.
Photos 5 that came with Catalina is the update to Photos 4. Possibly some preferences got disrupted in the process. iMovie only reads the Photos library that is designated as the system Photos library. To check, open your Photos app and click on Photos/Preferences/General and make sure that the box that says "use as system Photos library" is grayed out. Grayed out means that it already is your system Photos library and you need do nothing further. If it is not grayed out, click the box to gray it out. Then close Photos and close iMovie, and reopen iMovie. See if that solves it.
Also, perhaps your Mac has not yet finished updating your Photos library as a result of the Catalina upgrade. Depending on the size of your Photos library it might take a day or so.
Another thing that you can try is to open iMovie while holding down the option and command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. See if that solves it.
As a last resort, you might try redownloading iMovie.
-- Rich