yes it is true combined u0418+u0306 provides visually the same result..
and in my system it works as well (as long copy/paste didn’t trick me somehow). the combined character exist at least in char-map - the representation changes depending on following characters but I don’t know a way to translate the effectively used characters..
I created a combined character using charmap and used to rename the file using Web and Win desktop client - both directions worked and was reflected “on the other side”
Update: see Fixing the Cyrillic “й” filename issue in Nextcloud (NFC vs NFD) / Исправление проблемы с буквой «й» в именах файлов Nextcloud (NFC vs NFD) for details.