I'm afraid you've left out all kinds of important details, like the
exact version of PD365 you're using, what kind of transition did you choose (Prefix, Postfix, Cross or Overlap), how many transitions have you favorited, and how many clips are on your timeline. There might be a difference between having 5 clips and 100, for example.
I don't see any issues with any combination of types on v21.5.3125.1 with 25 timeline clips. I tried it with 3 and with 9 favorites and all work as expected.
It's possible your project has become corrupted, and one thing to try is to select all your timeline content (click on any object then use
Ctrl+A to select it all) then use File | New Workspace and paste everything to the new timeline. Save this project with a different name then see if the issue has gone away