Upon playing around further, I found a common denominator (at least in my case).
If I use the server’s IP address directly, it (re-)connects to the SMB share instantly.
If I use a custom hostname, it stalls the file explorer, until it eventually connects.
The only interesting entry in the logfile (log.smb) reads:
parse_dfs_path_strict: Hostname truenas.home is not ours.
But there is no issue with performance, file and folder access, browsing directories, transferring files, and so on. The only issue is the 20-30 second delay when (re-)connecting after some time has passed. (I’m not sure the log entry is even relevant.) After it connects (even if it hangs the file explorer for 30 seconds), the share works normally.
I tested this out on a new “Default preset” SMB share. Same exact behavior. The only relief comes from using the IP address directly.
This issue started after upgrading to TrueNAS Core 13.3-RELEASE.
With Core 13.0-U6.1, this never happened. Not even once.