After killing Cho’gall- the floor did not break and I was unable to kill Sinestra.
Same here. 25 Heroic.
The only time I’ve seen this is if you kill Cho’gall too fast. Which is admittedly almost guaranteed if you’re high level.
I haven’t tried it, but disarming and perhaps body pulling and love-tapping him so he can phase through his internal scripts a little might be needed.
If you have the toy, WHOLE BODY SHRINKA’, and hit it with before you engage Cho’gall you can autoattack him without one-shotting. Then you just click the buff to get back to normal and your good. (you might have to unequip weapon and reequip after depending on your level and squishiness)