Making my first mirror, and I've so far got the positioning correct, but the mirror seems to be mirroring items that are directly behind it (in addition to things that are in front of it, as it should), which I find kind of strange.

I tried editing the "bounding box" mesh just to see what would happen, but so far it only made things worse, so I went back to the unedited version of the bounding box.

Anyone has some clue as to what could be going on? I've looked pretty much everywhere, but can't seem to find a solution (everywhere I look seems to be talking about how to fix the positioning, not what to do if the reflections are wrong).

I cloned the BG "Atomic floor mirror," in case anyone knows if it's got any bugs like this.
(The mirror is part 2 of another object and needed to be freestanding, so it's not meant to be a wall mirror even if it looks that way)

I have troubles with mirrors quite a lot too so I know your pain. Maybe all the other reflections are showing because your mirror has all those extra sides picking up the sides and back of the object. I'd try getting rid of the sides and back and just leave the face Hopefully that'll work.

It only has the front set as a mirror (one side), so that isn't the problem.

The rest (backside + the ring around = 5 sides) is the "frame" part of the mirror. The square you see in the reflection is the textured back (which shouldn't show at the front of the mirror if reflections worked correctly).

Oh. In the second pic you have the little drawing saying the mirror is a 6-sided block so I thought it was a block instead of just a face as the mirror reflecting part. My next suggestion is off the wall, kind of, check the uv mapping. Are the parts that are making reflections that shouldn't be there part of the mirror or frame subset? Can you use the original mirror subset and just re-size it to fit your mesh?

The drawing was mainly to show the shape of the mirror, since it looks a bit like floating piece of "just mirror" in pictures.

The mirror part is separately UVmapped (haven't edited the UVs from the TS4 version I'm converting, but it has an UVmap that looks fine) and is linked to the mirror reflection material via the SHPE (haven't touched the original setup there), so I don't see how it could be picking up textures from the frame.

All the parts of the reflection that aren't supposed to be there are either part of the object itself (backside), or show if it's close to it - but not everything shows (the glasses on the pole shown in the picture show - the black "sticks" seen in the mirror - but not the pole, which is a bit strange - I at first thought it could have to do with TXMT settings, because the glasses are set to either "Blend" or "none+1" in the alpha settings due to some transparency (and this can cause all sorts of weirdness), but the frame part of the mirror doesn't have those settings, so I'm unsure if it can be something like that.

It's entirely possible it's a boundingbox issue (the - the mirror I used is quite a bit larger than the one I'm making - but most tutorials and tips say "just leave it" so I'm unsure. I tried editing it, and the result got significantly worse (reflections almost disappeared) so I don't know.