Thanks for your effort in isolating this!
Just tested this two different ways and yes, Create Widget using any CommonUI derived BP breaks everything and makes any other classes referencing it break too, so radioactive is accurate.
With my Canvas class being created it broke my Main and Pause Menu BPs as well.
With my Pause menu alone it only broke that BP.
This does mean CommonUI is useless for my project I guess.
Seems like we should file an actual bug report to Epic? Someone was saying that CommonUI was relatively abandoned, but I still see stuff for it on the Roadmap and it does work better than standard widgets, but I guess for now I will have to go back to default UI.