Changing fonts in Outlook - drafting messages

Recently I have found that Outlook changes the font on my. I have all my preferred fonts described in Options, Stationery and Fonts, but Outlook ignores this sometimes.
Example from today: I am writing an email in my standard font which is Calibri 11pt. I place my cursor at the start of the email to insert a new line, and the font for this new line is Aptos 12pt. Aptos 12pt doesn't feature at all in my stationery settings...

The same thing often happens when pasting into an email - even when I paste text only (which should have no format attached and therefore should use the existing format of the line I'm on...