It is a bit bugged. As far as I know enemies inside the rain when it stops lose the -res. If enemies have ran outside of the rain, the debuff lingers for a much longer time. I'm pretty sure this is technically a bug but it has never been fixed.
It sometimes lingers sometimes not, as far as I can tell.
But don't quote me on that cause I haven't exactly extensively tested it.
Culex's resistance guide
I think this may have read that this has been an issue for a while but does the debuff only last the duration of the rain (15 secs) when the power says 30 second duration? Whats the deal?