After update from SW 4.1 to 5.1 on Fenix 5x:
Bug1:
The first couple of minutes after the watch has been turned on, the timestamp of the first (oldest) sample in the pressure history is illegal (in my case it is a negative number!!). After this couple of mintes the iterator will return a valid first/oldest time stamp. Iterator-returning function attached.
Bug2:
Right after the SW update from 4.1 to 5.1 I had to wait 6 hours before being able to retrieve valid data from the pressure history buffer. I didn't do any debugging at that time and now I am not interested in going back and forth from 4.1 (since I will loose my settings) so I haven't reproduced the issue but I guess it has to do with the above mentioned Bug1.
Bug3:
In 4.1 the barometer time plot settings would reflect in the depth of the pressure history buffer (going up to 48 hours). In 5.1 only 6 hours af pressure history is available to the API no matter what the device settings are. The provided Garmin barometer widget is still working fine showing up to 48 hours of pressure history but the API is stock at only 6 hours of history.
I have not tested if the above iterator problems also applies to the other sensors.