Edit: Ok, we got a save state to isolate it.https://www.mediafire.com/file/4wfxuzk5gc998lh/Final_Fantasy_VI_T_Edition.000/file
Just miss Sketch on Hidon with Relm and it'll trigger. You will have to cancel the targeting, wait a few seconds, select Sketch and confirm. If it succeeds, reload, cancel Sketch, wait a few seconds more, try Sketch, etc. The screen will flicker and/or your inventory is going to get very corrupted. You must fail Sketch for it to work.
What our tester who got this working thinks:
It seems to be more transient than in vanilla. It doesn't happen at battle start, and it didn't happen after I used Ashura. It did happen after I used PuPu as well, though. I suspect it's some sort of VRAM overflow or something. My working theory is that using busy spells will eventually allow the bug to trigger in a longer battle. Other factors may include the business of the formation; Hidon's formation is one of the largest in the game. But it may be impossible to trigger the bug against e.g. a single Leafer.
Edit Again: Someone popped into discord and showed a small bug with equipped espers and the status menu. Equipping Asura for example here:
and then checking the status menu shows they have Mist Dragon equippd:
This happens for a few others like Pupu->Shiva etc.