Opening markdown files in iOS

I want to create, open and edit .md files on my iPhone.

I downloaded an app called Simpletext (https://simpletext.app/) to do this.

Simpletext won't allow me to create new files. I get this error message: "com.apple.DocumentManager error 1". I also can't open most .md files. Only two are available, the others are greyed out.

I think the problem started when I downloaded a second app, which is just called 'Markdown' in the app store.

When I have 'Markdown' installed, both this app and Simpletext allow me to open all the other .md files I have, and the two that I could previously open are greyed out instead. When I delete 'Markdown', then Simpletext will only allow me to open the two files.

The only difference I can find betweent the files is that if I go to 'Get Info', it shows the file kind as 'net.daring fireball.markdown' for all the files that open with 'Markdown'. The two files that open when I delete 'Markdown' show as kind 'Markdown content'.

I've tried resetting all the settings on my phone, and also factory resetting the entire phone, with no success.