We have had an issue occurring in a few of our groups where Yogg-Saron will spawn another set of tentacles on the transition to Phase 3. I haven’t noticed any reasoning behind when it happens - it’s not when you’re pushing brain room to the last second or anything like that. One kill we had a solid 25+ seconds left on the brain phase when it transitioned and we still got another set of tentacles spawning with the faceless guardians.
Have seen it happen in both 10m and 25m, only in the last 3 weeks or so.