Bear in mind we're just guessing here having not had the actual rifle in our hands. Also bear in mind no-one is "having a go" at you, we're just curious to find a definite answer to what & how this happened.
2 things that come to light from your post.
Firstly that is an abnormally high incidence of incidents.
Secondly there was a change made (the spring).
Based on that I'd guess there is a problem somewhere within the rifle/ammunition/user chain. I don't think it is drastic, but it IS causing incidents so it probably needs fixing for your comfort & safety. Ammo with 10 FPS is very, very minor, but ammo with a different pressure curve from a different propellant may well be causing the gas to behave differently, even if the velocities are identical.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”