Not too surprising, given that --expiry simply changes the definition of what a "stale" share is.
--expiry|-E
So with --expiry 1 you're defining any share as stale if it's older than 1 second. Pointless. And probably doesn't even have any effect, because you're submitting it anyway (like you should).
Yup. Seems my glee was shortlived. While I am not getting "mega stales", my rejections shot back up overnight. I must have made the tweak at the same time the pool changed to a slower coin. Gave me the impression I hit on my issue.
Back to the drawing board.