It's hard for me to say right now. I can say I have my difficulty setting manually set to 256 in my baikal miner (diff-multiplier 256), but currently we're on Dash in the auto-pool and my worker shows a difficulty of 1.58, and a rejected rate of ~1%. I almost always see 0% with anything except Dash-do you think this is because I've set my difficulty at 256, and, if I remove that 256 diff setting, will other, easier coins have the same issue?
I don't think x11 needs diff-multiplier settings, as x11 algo didn't have difficulty factor incompatibility issue from the start.
(Only qubit would need that option. Maybe it doesn't need that option at all. You can test it by changing diff-multiplier and see the reject rate from dashboard)
~1% reject is normal. Stale shares (little bit late share) can be rejected, and it can happen to any coin.
The reason why you see ~1% reject only from Dash is because Dash is the most profitable coin lately and pool mostly mine that. Other coins would not have long enough time to be mined.
1. Does diff-multiplier affects whole (X11, Qubit, Quark) algo settings?
2. Is your x11 hashrate reported correctly on pool even you set diff-multiplier? Then Baikal's diff-multiplier option doesn't seem to work with x11.