With no $filename() in the config, Tools | Options... | Export, [X] Write BOM fails to work.
Note: C:\Program Files\Mp3tag\help\options_export.html says
Write BOM
Mp3tag exports data in Unicode format encoded as UTF-8 by default. This option enables writing of a Byte Order Marker which is required by some applications.
This is mainly a documentation issue.
Initially I used UTF-8 as default but changed that due to many "Bug Reports" caused by third-party applications not supporting UTF-8 files. Obviously, I forgot to update the corresponding topic in the help.
Confirmed documentation fixed in V2.39n:
Write BOM
If you enabled writing of UTF-8 or UTF-16 via the $filename function in the export configuration, this option enables writing of a Byte Order Marker which is required by some applications.