Then I click “enter” to move a line down, the font changes.
This is the HTML of that note:
After you hit enter, please tap on </> - are you seeing the -apple-system being removed, or being added? It should be removed - the iOS webview inserts it and AnkiMobile needs to strip it out so that the font chosen in your template is used.
Apple systems being removed
And the font size also get reduced
Ok, that’s expected - it’s AnkiMobile removing styling that should not have been there in the first place.