Something strange is going on.
A fastmail account on one of my devices is definitely not doing it, I just checked.
Do you think you could 1) update to the latest development build from this forum 2) remove and add back the account in Aqua?
If the issue persists, please capture it in a debug log (see my signature), with "session data" in debug settings enabled.
With the logging turned on, please bring up "other folders..." 2 - 3 times, letting it completely refresh each time. Thanks in advance.