Me too! It is so frustrating! I know you posted this in March and its July now, but did you ever figure out a resolution? I've tried and can't find one.
I have the same issue, after my Samsung updated.
I went to Display Setting, and used the Screen time-out option, that you can change to allow your device lock within 30 seconds to 30min. That is the only way your phone won't stay unlocked until you lock itself.
I hope this helps.
This is unacceptable ! Cannot believe this happens on a 1000€ phone.
Samsung, please solve this !