iOS mail only displays plain text

My iphone is displaying emails in plain-text only. My iPad will display the same message in HTML. I can’t figure out what is wrong. I’ve tried removing and re-adding my accounts. What else can I do?

Have you swiped-up in card view to quit the app?
Have you tried restarting your iPhone?

In earlier days you could quit apps while in wiggly-app-icon delete-mode in the springboard: there would appear a minus-sign for running apps in addition to the ‘x’ for deleting apps, and clicking the minus-sign would quit the running app. Once, years ago, I had the same problem as you, and simply quitting the app fixed the issue.

I have tried force quitting the app and hard-resetting my phone with no success.