Poor original installation, improper epoxy prep and two-part mix or too many backfires.
I’d call PP and see what “epoxy” they recommend since they sell the valves and provide tech article.
That said a new surface prep will be required, whether this can be accomplished without taking air box off but the old epoxy will need to be removed.
It’s important to know the makeup of the parts you’re gluing together, epoxy works wonders but doesn’t set up properly if it isn’t mixed correctly. Epoxy sticks to most everything but needs “teeth” to hold, if the air box is ABS, polymer based or the ubiquitous polypropylene needs to be determined when using adhesives other than what’s recommended. The terms “plastic and epoxy” are used loosely and often generic.
https://www.pelicanparts.com/techarticles/911_pop_off_valve/911_pop_off_valve.htm