On my non-plus Fenix5 the backlight worked just fine with gesture during an activity (run)
On the 5 Plus the backlight never comes on, even though "do not disturb" is definitely off and not within sleep hours.
Not during activity it seems to work, kind of, the sensitivity is very poor compared to the non-plus
Garmin has a "power management" setting with a "jacket mode" but not the 5, only the 6 and newer
Also cheaper/lower-end Garmin watches can adjust the sensitivity but not the flagship Fenix?
Read a theory elsewhere that when the wrist optical HR is turned off there is a bug that the gesture sensitivity is buggy
I do use an external HRM strap (HRM-TRI) so maybe that is yet another bug?
Probably not enough viewers to confirm but if anyone can in the future feel free to reply.