Same issues. When I burned away the webs, there was a nice animation of them disappearing, but the spider (both the matriarch, and a regular phase spider) didn't fall. The matriarch actually showed a little line of dialog by it's head, something like "hum...", I forget exactly, but it acknowledged something happened. I didn't break stealth, so it kept strolling on the non-existent web.
Also when I killed both spiders, who were standing on a destroyed web when they died, they both fell to the ground below, as you would expect. However, when I went to loot them, the engine walked me back up to the level of the destroyed web and out on the non-existent web, and then looted the body.
I also thought it was strange no spiderlings spawned during my combat?! I destroyed the eggs near the Matriarch, but only that one clutch. I didn't happen to look to see if the others were still there, or if they were removed. I was able to land Laezel's menacing attack each round, so maybe that's why? The matriarch did attack from range, but never moved, and didn't hatch any eggs. It was a very different combat then my previous (6 or so) playthroughs.
(We won, but just barely. Gale was the last one standing with like 2 hp. THAT'S a first.