auto retuns to activity mode after "resume later"

Sometimes it happens to me that my watch vibrates and is in running activity as I pressed start button by accident by bending my wrist or pulling my pullover. Maybe the same for you.
You could use key lock and see if it is happening then too.

For example I have 2 rides per day. [..] When I finish ride TO, I press stop and resume later. Then during in day I catch that my watch not in watchface mode. it turns activity mode on, and stay on pause (gps is on).

There is a maximum time that the activity can be paused using Resume Later. I am not sure how much time can pass between the two parts of an activity on the FR935, but on my Fenix 3 it was 8 hours, and then it would auto-save the activity if not started again. I believe you could do a Resume Later for a second time, but am not sure how the recent software versions handle that.

I sure this is not normal process. and I disagree that I pressed start button by accident. I try to do this - no any chance. I never get "start button press" when watch not in "resume later" mode.
Today I catch return after about 3.5 hours after I paused ride. my hands were on laptop keyboard - its impossible to press start button... I think watch returns to activity after notification from app/sms (sometimes).

I believe you could do a Resume Later for a second time, but am not sure how the recent software versions handle that.

yes, I can press resume later again. but returns to activity almost silent, and while I don't press "resume later" again, gps is turn on...

After comment I put my watch on table. After one of the notification (it was from Gmail) - watch does not show notification, but returns to activity.

My FR935 does not do this. Maybe something is wrong with the activity you are using, perhaps it helps to reset it: Settings - apps and activities - (activity) - restore defaults. If you are using a custom watchface, try what happens with one of the default built-in watchfaces. Otherwise contact Garmin support. They will probably ask you to do a full reset of the watch, so that is something you could also try beforehand.

Did you ever discover the solution to this problem? My 935 is doing the same thing. I had the buttons locked last night and when I woke up the watch is in activity mode.

My Fenix 5 Plus do exactly the same thing every time, also my old Fenix 3! Something is defiantly wrong here. Garmin need to adress this in a update.

I just posted a new thread for more or less the same issue (out of two). The issue is that if u have a to-be-resumed-later activity stashed in background and you let a widget time out on its own then it would time out and jump not back HOME but to your to-be-resumed activity. Even worse so, from there on, at least my FR935 does not follow what the Power save timeout (Normal) is telling it to do but instead stays in activity mode forever and just kills battery. Awesome, Garmin :) Mere 4+ years since this post and yet nothing changed.

https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-935/295262/1-widgets-time-out-auto-return-into-a-to-be-resumed-later-activity-and-not-to-home-watchface-2-activity-power-save-timeout-not-working