HR bug - time in zone GC app not reflecting %LTHR

My average HR should be in zone 2 if you look at my run recap on the app. It's look like I have been running in zone 3-4-5. Quite impossible since more than 50% was in zone 2. My zone 5 start at 186bpm as you can see on the watch i never zase close to my red zone.

How can we explained the discrepancies between the app and the watch graph. This impact massively the volume of training calculated by garmin in my run recap. This should have been easy not a threshold session at all.