They actually fixed this, the ice doesn’t hurt you during Ymiron’s stuns anymore.
It’s no longer a problem, but for all of phase 2 when titan rune dungeons were first released, web wraps dropped threat. If the tank got web wrapped while Anub was casting Pound, he would instantly turn around and one shot the group. The mobs at the start could also easily insta-gib you in the split second threat drop (because you have to be stacked in melee to break webs fast). They fixed this, and the stunned-in-ice issue, with the release of beta. But they left these glaring issues for entire phases before fixing them. Frustrating.
You can see the changes on July 6th and 11th, 2023. They also made the loa blood healing one way less annoying at some point.