I searched, but none of the many pages of results were relevant, and restricting misinterpretation of the search criteria reduced the results to zero.
I recently placed the base NPC "Midnighter Loremaster" in my base. Allegedly this NPC is a "trainer", which I took to mean you can interact with it to level up, change active build, and in some cases choose a title or correct the colors of the title you already have (don't get me started on that bug). I previously used the Vanguard NPC trainer in another base, and it works just fine.
The Midnighter Loremaster, however, is broken. You can highlight it, but it is not interactive. It does not matter where you place it (near a wall, in the middle of open floor, etc.), when you click on it it becomes highlighted. And that's all. It is not interactive, the cursor is always a white or red arrow and never a blue hand, and it cannot be used like an object that meets the standard definition of "trainer".
Workaround? Known issue? The NPC was placed three days ago, so yes, I have quit and relaunched, and quit and rebooted and relaunched, multiple times without any change in its total lack of functionality. I have also sold it and repurchased it without any change in the lack of functionality.
I haven't tried either of the redside NPC trainers to see if they work at all.