I forgot to mention, but its probably relevant:
After I forced theme, visitor is redirected to the URL without parameter.
So if page URL on mobile device was for instance:
https://domain.com/page (mobile)
I clicked “force destkop”:
https://domain.com/page?am_force_theme_layout=desktop
I see mobile version again at:
https://domain.com/page (should be desktop now)
but then if I go to other page with my mobile, it has been changed to desktop correctly:
https://domain.com/page2 (desktop)
but if I force mobile again here:
https://domain.com/page2?am_force_theme_layout=mobile
it shows desktop version anyway…..
(hope it clarifies the situation)