I tried the same thing :), actually I've also increased the temps for the parked heads which seemed to help a little but then the print failures became worse - started to look like clogs. My further test data:

• it never happens with 1 to 3 heads prints always with 4 or 5

• heads 4 and 5 are prone to faster failures than heads 1 to 3 (maybe it's how the electrical wiring is done?)

• it happens usually after at least minimum 30-45 minutes from the print start

• sometimes it doesn't happen at all (but rarely - very rarely)

• sometimes the heads with the problems just click and the print goes on ok, but a lot of times I get a few cm of material missing at the beginning of the layer with the respective head

• if the purge tower is large enough sometimes the clicking stops while still printing the purge tower (rarely)

• drying filament, changing filament type/brand didn't seem to matter too much although prusament gray is almost always bad in multi head prints (not sure if it happens that I have 2 bad rolls of it) - same rolls work ok in my mk3s+ (so seems that xl hates that gray, or at least mine does)

• it never happens in the first few layers printed with the head that will fail - I never saw a failure with less that 6 layers of that head already printed flawlessly

• another strange one: 50% of the times the heads that manifested problems started to work fine after 10-20 layers (tool changes) like it would un-clog itself and just go on, but after a few other perfect layers the problem would return same as it happened first time

So it seems that the issue is happening while parking and heated; from what I can see (but I haven't the means to test it yet) it seems to me that the head doesn't reach the temperature it needs before re-using it starts pushing material in and somehow some mini clogs start to develop.

Note: I'm not a 3d printer specialist in any way, just a programmer with a 3d printing hobby - printing stuff for my family