I have no idea if someone ever solved this bug, due that the last published report that I got to found was in 2020 but there's this annoying bug in Old King (Chaos dungeon depth 9 boss) Where in the 2th phase you cannot break his mirror copies, making that you cannot beat the boss, this is because a variable in the save file (#121) that is in charge of the break effect on the old king copies is stated as 0, you have to change it to 5 or more value to make possible his break (this is a alternative for the interchange of save files or start a new whole file for the value to not be in 0), the file is crypted so you cannot change it normally, I recommend this page, it works really fine for that:
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Pd: Something very important is that the variables only got registered in the save file once you fight the boss, so, you have to lost one time for the variable to appear and then you change it's value, I hope this is useful for someone struggling with this bug