This used to happen a while ago, and now started to happen again with 2.4.1, haven't tested 2.4.2 yet.
To replicate, select a performance with all 5 slots filled, edit #4, press "store" 3 times and sooner or later you'll notice that slot #1 has the same rig as slot #4. Haven't tried with other slots besides #4, but this one was the one that used to overwrite #1 when I noticed this bug for the first time, a few firmwares back.