Thanks. It sounds like you've got a protocol which should make it possible to reproduce your observations. Next time you observe the behavior (or this time, if you still have the browser windows open from before), could you please note, and report here, two items of information: (1) the thread (url) where you see the problem happening; and (2) more importantly, the specific comments (by node_id) that appear in the logged-in view of the thread but not in the logged-out view. Just a handful should suffice, if the number of comments in that set is large. Thanks!
I reckon we are the only monastery ever to have a dungeon stuffed with 16,