I recorded a video of what is going on in 4.18.1 with a new blueprint TPS project.
https://www.youtube.com/watch?v=TyKk2IJf6gE
If Enable Window Animations is enabled, opening a Class drop down list that goes off the bottom of the screen will rapidly flip its anchor making it unusable. Having any (Class, Asset or Collision Preset for example) list open and scrolling the panel the list is part of up/down with the mouse wheel will also cause something similar.
If Enable Window Animations is disabled it will open properly. Scrolling the panel the list is part of with it open also works
Tested in a new and old project, versions 4.18.1 and promoted from the 25th.