Hi there, @MikeWeston ...
I had to do some checking with Instructure staff on this. While there doesn't seem to be a Guide that indicates what the default font is in the Rich Content Editor, there is a Release Notes document that describes that the Lato font is the default:
Canvas Production Release Notes (2016-12-10) - Instructure Community (canvaslms.com)
Specifically, check out this section: Canvas Production Release Notes (2016-12-10) - Instructure Community (canvaslms.com) -- User Interfa...
When you say that you pasted in some text from another source, what source was that? Were you copying text from a Microsoft Word document, for example? Sometimes when pasting text from another source, you can get a lot of unwanted HTML code (I refer to this as "rogue" code). Sometimes, this also includes HTML font elements that are unwanted. Are you wanting things to display in Lato font (the default)? You could try running your page through an online HTML cleaner if this is what you are wanting. A long time ago, I wrote a short blog on the subject:
HTML Cleanup
If you could provide a bit more detail on what you are trying to do, I think that would be helpful. Looking forward to hearing back from you soon!