I was looking to see if someone had commented on this. For me it sometimes even happens if I start the workout before starting navigation – it will start off in the right configuration but then suddenly switch back to workout data in the middle of the ride.

Google Maps on the iPhone has a similar problem where a banner notification can block the section at the top that shows the next turn. If it's persistent (e.g., a calendar reminder), you have to try to swipe it away while driving without clicking on it by mistake. I guarantee multiple people have crashed because of this.

Whoever at Apple thinks that anything at all should override navigation for more than a couple of seconds without explicit user action is an idiot.