Log rotation has been a feature of Joplin since v2.12.
Logs used to get a lot larger than 55MB. From my system it looks as if the log gets truncated at ≈100MB.
The release info states:
Rotating log
Up until now, the logs could grow to become very large, and if you wanted to shrink them down you would have to manually delete them. With this new release, we now automatically handle logs by rotating them - once the log becomes large enough it is moved to an archive. And once the archive is older than a number of days, it is deleted.