Hi, BJArtist,
There have been several reports on this board of transitions disappearing after the recent update. It likely is related to the update and so Apple would need to address it.
However, you can try the following diagnostic procedures to see if any one of them will help.
Deleting Preferences
Try opening 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.
New Project
Another thing you can try is to copy your media into a newly created project and see if
that cures the problem. To do that, do an Edit/Select All, Edit/Copy, of the
media in your project and then Edit/Paste it into the newly created project.
Safe Mode
You could also try booting up in Safe Mode (see Finder menu for instructions) and,
after everything loads, immediately shut down and reboot in normal mode. Then
see if iMovie works properly.
Resetting the System Management Controller
Try resetting the System Management Controller:
https://support.apple.com/en-us/HT201295
Resetting the NVRAM or PRAM
Try resetting the NVRAM or PRAM, although this issue would be fairly rare:
https://support.apple.com/en-us/HT204063
-- Rich