Really an oversimplification / misunderstanding of the mechanics. We did some exploration and its actually a memory corruption glitch. It turns out its not even related to clogging up all pathable tiles- placing enemies / pathfinding / updating collision all work in cases where they don't resolve to find a valid new location because it either terminates on fully blocked terrain (islands) or maxes out the iterator. Instead its simply an overflow with the collision size of the portal and how many tiles its supposed to be occupying when the iterator checks collision against its tiles. Its attempting to check collision against more tiles than the unsigned integer limit and for whatever reason this causes some line in that one update function that gets invoked by everything to use "<<<<<...." several times to bitshift memory belonging to monsters, over and over. One of those things where the line is supposed to shift memory, then unshift it, but one fails. The reason monsters are dying isn't even from taking damage, their life is among the monster data being shifted, apparently about ~20 times per being invoked hence anything with less than 1048576 hp is being killed instantly, unless the next patch of memory wasn't blank it might even give them millions of hp (and might kill them on next update). Not sure how they arrived at 2
but that matches up with testing, P8 baal was sometimes just losing a huge chunk of his HP when he got 'hit' or just suddenly died like he got 2-3 hit.
But its still only apparently applying for collision against the ridiculously oversized town portal so yeah you need to spam it to a critical mass to trigger the overflow, then each cast after that is still exponentially raising its size to check collision for monsters as you teleport away, and if you do it too much you'll either crash the game (online) or fry your computer (offline- do not recommend). And notably, the whole thing only works on these long teleport paths like spider forest -> travincal because town portals are remaining 'active' regardless of your distance from them, they are still loaded in as actors even if you're 3+ screens away because they're supposed to actually keep the area around them loaded.
This also explains some of the other weird effects we've seen like corrupted droprates and TC tables. I have no idea what order the memory is in or getting jumbled so the effects could be really nonobvious, we just all noticed enemies dying in 1 hit presumably because whatever memory is next to life is usually set to 0000 and its effectively nuking them for 1 million damage a pop