I have been using canvas for years at my university and I have never had this issue.  For all my Canvas quizzes, I put a time limit.  A student emailed me about a final they were taking saying that instead of getting a countdown timer, they got a message in red that said their browser was slow and it was removing the timer and giving them 5 extra minutes.  Like I said, I have been using Canvas for years, and this has never happened.  I reached out to a bunch of different people including Canvas support.  So I was figuring that student was making it up.  But then I was able to recreate it!  I had a few tabs open in Chrome and I got the message where the timer was that said in red "Your browser connectivity may be slow or unstable.  In spite of your browser's timer being disconnected, your answers will be recorded for an additional 5 minutes beyond the original time limit on this attempt."

Giving five minutes to students without my consent or knowledge makes me feel uncomfortable.  I also don't like that I don't know which students receive the message and additional time.  If a student is given additional time, shouldn't the instructor be informed?  I don't like it.  I also don't like no one knowing anything about it.  This sort of thing seems like something we should know.  But I don't know what can be done.  Any thoughts would be appreciated.

I guess the most helpful thing is acknowledging that it isn't necessarily a bogus thing students are saying.  I was able to observe it in a preview for a quiz.