Update: As of 6/28/2018, while we have not received official word from Canvas, it does appear that this issue has been resolved. 

A Canvas styling issue was reported to us earlier today. If you add an H3 to any assignment, discussion, or quiz the text will turn blue and it will reduce the size of the font when saved due to a very broad CSS declaration from a learning_outcomes.css file loaded by Canvas. We have contacted Canvas and they are working to resolve the issue.