I has added the custom filetype into kwrite when the old version, and noticed it's ignored by the recent version.

How to reproduce:
Open kwrite and it's menu, "Settings" -> "Configure Editor..." -> "Open/Save" -> "Modes & Filetypes".
Push "New" button to add "New Filetype", and do "Apply", "OK".

Then,

Reopen the setting dialog. "New Filetype" will vanished from the drop-down list.
Click of the right of the status bar, and search "New Filetype", but it will be not found.
Check .config/katemoderc. "[New Filetype]" will be added.

So probably it's only kwrite ignores the entry of katemoderc.
Are your custom filetypes working?

Thanks.

kwrite version: 20.04.0
OS: KDE neon 5.18 (I've updated all)

It seems the related bug:

417507 – Kate ignores new mode/filetypes
https://bugs.kde.org/show_bug.cgi?id=417507