Thunderbird "Empty Trash" causes inconsistent IMAP session state?

[awl1]

All,

having successfully compiled and set up Dovecot 2.2.29.1 on my Thecus 
NAS as a newbie without any further hassle, and already imported an 
external mail archive of ~15 GB into a lz4-compressed mdbox (with 
impressive performance on the old Intel Atom CPU!), I stumbled into a 
minor, but reproducible issue that might well be already known, but I 
haven't managed to find any pointers through Google search:

When I *manually empty the Trash folder from Thunderbird* (52.1.1, 
regardless of Windows or Linux client) by right-clicking on a non-empty 
Trash folder and then selecting "Empty trash", Dovecot produces the 
following log message, and I need to restart Thunderbird client in order 
to properly refresh its folder status:

May 30 23:42:04 imap(xxx at xxx.org): Info: *IMAP session state is 
inconsistent, please relogin.* in=405 out=3114

Is this behaviour and message intended, or did I indeed run into a bug here?

Many thanks & best regards
Andreas