Heres what I determeind with some research. The action lock unlock after fireing to facilitate faster pumping (having to hit the release isnt practical). The recoil knocks it back. That is why you dont notice on light loads (the light recoil). It happens with both manufactures.
From Remingtons website-
The action on my Model 870 opens when I fire it. Is this normal?
Question
The action on my Model 870 opens when I fire it. Is this normal?
Answer
It is the design of the Model 870 for the action bar lock to disengage once the shell has been fired. This enables you to load and fire the second shot with more speed.
If you have any concerns regarding the safe operation of your firearm, you can take it to your closest Remington Authorized Repair Center for evaluation.
From Mossbergs
NOTE: During firing, the action unlocks automatically after the hammer falls and
the action can be cycled without depressing the action lock lever.