Oh dear.
Double facepalm.
So how should we bug this one? Sense Weakness prevents critical hits from enemies? It's kind of a whopper if we are understanding this correctly.
Edit: Attempt at an entry.
- Sense Weakness bug appears to prevent enemy critical hits: A look at the sense weakness code (link to thread) reveals an error where sense weakness is effectively preventing enemies from inflicting critical hits - but only if you don't have any ranks in the skill. When they would normally inflict a crit you instead see a "sense weakness" mention in the combat log and the crit is averted. Some players are also reporting they never been hit with a crit.
I'll just replace what I have in the thread now with this.