There should be some sort of tag for "panties covering genitals with anus prominent".

isn't there a "no back/ass panties" tag?

There's "backless_panties" but that shouldn't be used for pictures like this, because it refers to panties intentionally designed when worn PROPERLY to leave the wearer's anus exposed while covering the vagina.

That's obviously not what's happening here. I'd love a tag for it but can't think of anything snappy to call it.

panty_malfunction?

How about ass peek as a tag?