@feditips
I tried to reproduce the problem last night (leaving the tab open and idle for several hours) with the console open including the "network" option enabled. It did not recur.
But today there were not any omitted ("...") posts, either, which was part of what happened yesterday. (I don't really think that is likely to have had anything to do with it, but I don't know)
Until it happens again I'll assume it was a transient problem, probably load-related.