I assume you are referring to the scenarios delivered with BTS, which are essentially mods, yes. These mods will not work with BUG, so if you have that installed that may be your problem. Furthmore, I think BUG works with 3.19 only now...not sure on that one.
Kinda odd that you would post from the standpoint that the delivered scenarios are broken as opposed to maybe it's a problem on your end. Otherwise, you probably would have easily found that issue here...it does not exist.
However, the missing interface issue has come up before and it is generally related to a bad install or conflicting installed mods.
If you install BUG into custom assets (SP mode) this will conflict with just about any modpack - delivered or otherwise. However, SP mode BUG is the best and easiest way to install it. So what I do is create a copy of the Custom Assets folder and rename it to something like "CustomAssets-BUG". A new vanilla custom asset folder will be generated automatically when you load up the game again. Save a copy off of the vanilla CA folder. Switch between the two depending on whether you are playing a modded or unmodded game. (Bug in CA will work with some scenarios like Earth18 since they are just maps)
I actually use a little app that lets me switch between CA folders in a snap. However, you can just rename the folders pretty easily to get the same result. Choose "Rename" from the right-click context menu and delete "-BUG' from the name to make it the live CustomAsset folder, but add "-Vanilla' to current one before doing that. Reason I use the tool, besides making it easier and quicker to do, is that I always know what is in the current live CA folder.