Appears that the Iris PC-21 HUD gets wet inside the cockpit when it rains, making it unreadable in instrument conditions (this according to dialog exchange on the Iris Discord channel.) Developer responded with, "lol, yeah, that's the limitation with XML gauges. We're working on a HTML HUD, but for the moment, use the F-18"
Default F-18 HUD also gets 'wet' and unreadable when it rains, btw.
I just can't deal with this kind of mentality. Bang, erase, lesson learned - Iris hasn't changed much since the old days (sigh). India Foxtrot Echo, count me in.