It's been a while since I've delved into the specifics myself, and I'm not entirely sure if I fully understand the issue, but from what I can make out, there's some sort of incompatibility between tModLoader and vanilla Terraria over the data structure that maintains the Big List(TM) of all the items in the game, which manifests in unusual behavior when modded items are placed on display items like item frames, mannequins, etc. I've seen world files get corrupted because of this issue, so it's no joke.
Unfortunately, as you said, it's a long-standing issue that has resisted all efforts to fix it thus far (which is why I'm starting to tire of seeing this particular "bug report" in every
ing mod thread every three or four days), which is why I'm not confident it'll be resolved to everyone's satisfaction anytime soon.