German special characters like umlauts (ä, ö, ü) are not converted correctly with convertFilenamesToAscii Filenames with German special characters are not converted correctly with convertFilenamesToAscii, e.g. ä, ö, ü are converted in a, o, u in contrast to ae, oe, ue in slugs with limitAutoSlugsToAscii.
This was a bug and has now been fixed in Craft 3.5.12.1
https://github.com/craftcms/cms/issues/6923