I can confirm this bug - it happened on my most recent playthrough (Seldarine Drow dual-weapon fighter, with Lae'zel, Shadowheart, and Gale - all level 3). In the past I was able to firebomb the web the Phase Spider Queen was standing on, and she took a good chunk of damage, but this time out, I had to do it the hard way.
(We won, but just barely. Gale was the last one standing with like 2 hp. THAT'S a first.
Additionally, characters who don't have the spider boots will become enwebbed near the destroyed webs.