See this topic.
I turned back to v1.2.1.5 and tested it again. This mechanism/interaction only exists in playtest.
It must be a bug. Enemies already have deadly debuffs, and the values are not acceptable if enhanced by Sunder. In my previous post, two screenshots showed 71% Fumble and 96% RD, 96%! Changelog didn’t mention annything about this.
This bug significantly increases the pressure/difficulty of survival. All builds, especially those relying on leech, become more squishy and fragile. We cannot make a correct evaluation of builds’ survival ability in playtest.