This is because you, probably, didn’t mirror UV.
To do that in Mirror Modifier > Data check ‘Mirror U’ to mirror horizontally (‘Mirror V’ - vertically).
It’s this overlap that is confusing me … no idea where that comes from …
Also I don’t understand why the Team Crest is being mirrored.
I think the issues I have are caused by the UV maps … I’m now doing the long workaround by creating new UV maps after having applied the mirror modifier. Would still be curious about the correct way of handling this tho.
I don’t have any problems with Team Crest in this file, so can’t help you there, sorry. (using Version 2.92.0, but not sure if it’s relevant)
Overlap (and other strangeness) must’ve come from some editing along the way.
You need to move the half so it’s center-line falls straight in the middle of the map (or use Mirror Mod offset setting, but that’s whole other problem). I haven’t found a good way to do this, but here’s what can be done:
place 2d cursor at the center: N-panel > View > 2D Cursor > Location: 0.5, 0.5;
move island so it’s center-line is at the cursor (more or less);
set pivot point to 2D Cursor, select all the vertices of the center-line and scale with S > X > 0 (to straighten out the line AND place it precisely in the middle).
This should insure it’ll properly merge when you apply Mirror Mod.