Nav VI dashboard display has a mind of its own

[Hors Piste]

I have set the left hand dashboard display panel to display the ambient temperature (only works when on the bike; off the bike there is an option to display "Temperature" but nobody seems to know what that is). This works fine, except when I start to follow an active route the panel automatically switches itself to "arrival time".
I can get back to the Ambient temperature by pressing the screen and going through all the options again but that's a pain.
I found this out by playing in the garage just now and setting a route active. Ambient temperature 26.5, pressing "Go" changed the display to arrival time, scrolling through the options I found that the "Temperature" display was reading 21 - I wonder what/where that is? - and finally got back to Ambient temp which is where I started.
Garmin/BMW just being "helpful" I suppose...

another edit - I think the 21 is the temperature in the locality, being picked up from my phone via smartlink when the nav is on the bike.  26.5 was the temp in the garage - although quite what the difference is between "temp" and "ambient temp" then is I'm not sure....