I reported this issue back in September and it didn’t gain any traction even though it was fairly reliable to reproduce for me (always happening after my first log-in for a game session). The workaround I came up with was attempting to scroll the guild log’s window which forces a retrieval of the chat (though sometimes this still doesn’t work).
Sounds a bit like something was changed which is making this occur for far more people.
EDIT: I see this is also about roster, which our guild currently isn’t experiencing. Still just the random chat blank issue for us.