This is a well know bug in FF3.
In this game, you can get three 'degree' of ending.
1 - Just after beating Yamazaki
2 - after beating Jin Chonshuu
3 - after beating Jin Chonrei
The ending you get is clarely based on the level of your fighting performance throughout the game of course.
Now, when you meet the FIRST Jin brother (Chonshuu), the case if you're or not getting the second one as a final boss is determinated ALREADY. In fact, you maya ctually fight against Chonshuu in TWO different stages: 'actual' Delta Park (the one with woods and all the rest) or 'psycho' Delta Park (the one made by flames).
If you're in the first one, the game is already supposed to end after you beat Chonshuu, if you're in se second one, after beating Chonshuu, Chonrei is supposed to chanllenge you in a colour palette changed stage.
Now, for some reason, when you beat Chonshuu in the first case (thus leading to the ending), you get the beginning of the demo cut introducing Chonrei (which should NOT appear) and the game freeze in this bugged situation.
I don't know if this occur with all character or just with some of them.
Terry used to encounter thid bug in the MVS, AES or even emulation.
Dunno if ALL copy of the game are bugged or if latest copies were fixed, but I actually doubt about this second option.