My test file was called HòcusPòcusTest.png.

OK this may be caused by a wrong LC_TYPE not compatible with accents. I say this thinking about MediaWiki that probably is calling escapeshellarg() to sanitize the filename argument before passing it to /usr/bin/convert but that function is known to occasionally cause artifacts if you do not set your correct locale:

https://www.php.net/manual/en/function.escapeshellarg.php#99213

I was able to reproduce the problem in another PHP website. From command line it was already working fine (since I have all the LC_* stuff) but from the website itself the accents were dropped. I've fixed adopting this:

setlocale( LC_CTYPE, 'C.UTF-8' );