Of course there is the other thing of course, even if the spell "flickers" and needs to effect you again, it will effect you in the short time it takes to recast your parma.
Oooh, nasty idea for an item, it triggers exactly at dusk/dawn when the Parma will drop, so it can effect you when you are defenseless.
Hmmm- I agree it's not inarguable, but the word "truly" seems to be the clincher for me. A "truly constant effect" is not the same as an "effectively constant effect", which is what you're talking about.
"Truly constant" means unwavering, unstopping, unchanging- no flicker of the spell, no flicker to lose penetration.
If it had to be renewed in any sense, it wouldn't be "truly constant", now, would it??? :\
That would be a fantastic secret for a hermetic traitor to leak to the opposition.