I’m using 11.2.5 on a 7plus. Just set an alarm to go off a minute ago. Phone was laying down, screen off.
Alarm went off and all I did was tap the stop button (did not lift the phone up at all) & the alarm silenced as normal.
Have you tried a reboot of the phone itself? When strange things happen, that’s the first thing I do and it seems to fix 99% of the problems.
I’ll update to 11.2.6 soon and see if it changes the stop feature.
EDIT: just updated to 11.2.6 and can still shut off an alarm by only pressing stop on the screen.