Necromancer Lich form stability has not protected me from knockdowns or knockbacks in numerous fights. It may be a problem with the new stack of stability not refreshing after the old one expired.
I have observed this with a veteran wurm, which has a single knockdown skill, but also in fights like Wyvern Patriarch and other wyvern fights in HoT areas. I have not seen this happen in Gorseval fights with her ground slam, however.
Attached is a screenshot of my character knocked down by the wurm’s missile attack. It occurred approximately 3 seconds after I cast Lich form, enough for me to summon jagged horrors.
