Default transitions dissapear after Golden Gate 27.0.1 and iMovie 10.4.5 update.
Creating a new movie project or opening an existent one, and moving to the Transitions tab, it shows nothing. Already created movies still has the transitions, but if you want to create a new one, can not.
This starts happening after 10.4.5 update, with 10.4.4 it works perfectly.