Interesting, so you know all the basics already.
Taking a look, "Filthy Look" and "Dirty Grin" are the same skill/attack. Nearly 100% to hit if used, and raises submission by a certain amount as noted. On the default version of the hyena thugs, the grin attack is present, but lower priority than the basic swipe attack, so it should happen every so often, not constantly.
Are the hyenas doing their normal attack at all? If something has somehow triggered disabling that, they'll default to only the grin as that's all they'll have, but I'm not sure what would cause that. Check that you're on the most recent version, and upload a copy of a save it's happening on to catbox.moe for me to look at. Also, sometimes people will just get very odd RNG or weird glitches for reasons I've never been able to pin down due to a "bad save". If possible, I'd try replaying from a bit earlier (say a save right before Mezz is knocked out in the barrens, or even just when you first enter the district) and seeing if you still encounter the grin behavior on that save.
I will say that yes, on a totally fresh new-game encounter, first mob in the first district, NORMAL hyenas, Mezz should not encounter a guaranteed series of grins (which would indeed lead to rapid max sub and quick defeat. That's not intended behavior. However, "after climbing the fire exits" implies you're actually about halfway through District 1, not just starting it. Have you gone through the tunnel and out the other side? In that case you'll be fighting some specialized hyenas, not the normal thugs, which does shift things a bit (although even then, it shouldn't be a constant string of grin attacks).