BUG: Cant go back to home screen

Have you disabled lap key, in settings for that activity?

Is this the long press back button? I thought this was a hot key setting exclusive to the Fenix and 9xx lines. Would be great if Garmin has made this a general feature of all watches. 

I only have 245 and what back buttons does, depends whether lap key settings is enabled or not on that specific activities. On some activities I use it for manual lap on other I use it to see my custom watch face.

The FR255 haven't hot keys, according to Garmin website. Try to compare 955 vs 255 and show the Hot Keys section, for 255 is empty.

I have the same issue. There is a workaround though, you can hold the up button to get to the settings menu -> appearance -> watch face, then press the back button and you will be back on the home screen. Very clunky and unintuitive, they should at least add it as an option in the controls menu.

i have a 955, but i wanted to issue a warning...

unlike my 935, my previous 945LTE and now my 955, when exiting out to the watch face/glances using a hotkey shuts off the GPS radio. the activity keeps logging but the GPS is lost. i've given up fighting Garmin on this, but it's frustrating because i too like to use Glances while hiking yet don't want to lose tracking info.

Is there any news after 6 month?

Nope, still works on the 245, but not on the 255.

Serious question - why do you need the main screen when you start an activity? If I need a break, I do  pause/stop the activity?