Font keeps resetting

Hi again!
So this morning I was working on a new book, in a completely different series than the one I’ve actually finished and sent to my test readers. And so far, the program is working exactly how I want it to, with one small exception.
When I hit Shift+Home (to target the entire line of text) and then backspace, it changes the font from my preferred Courier New to Arial. I already went into Styles and changed EVERYTHING to use Courier; but this keeps happening until I backspace again and create a new line again.
Any thoughts?

  • ME

Does this happen regardless of where in a paragraph you place the cursor?
Are you using styles consistently?
When this happens, is the style you chose for the paragraph still active or does it say ‘No Style?’

The only thing I can think of is that by deleting the whole line you’re somehow losing the style in the paragraph. I haven’t been able to reproduce it though.