When you get home, do you jack your car up to take pressure off the suspension?
Suspensions needing to be replaced is due to all of the work that they do compressing and decompressing, not due to a constant pressure.
Just to toss this out there... Not all springs are created equal.
I have a Mossberg 500A. I used it for a home defense gun for a while; I kept it topped off, and it sat fully loaded for 5-7 years with no use. I finally unloaded it and discovered that the last round would not get delivered to the elevator completely. The spring wouldnt send the follower all the way home. I even went so far as to buy a new spring. Unfortunately for me, the magazine tube was a bit too tight and I couldnt get it off without fear of damage, so I let it be.
Fortunately, leaving it in the safe unloaded for several years allowed the spring to return to full function and deliver that last round reliably.
Granted, tube springs are a different animal than magazine springs, but in certain circumstances you can over-compress a spring.
I do keep my AR mags topped off, regardless of my Mossberg experience.
I've acutally got the exact same problem with my 590. I keep 1 in the snout and 8 in the pipe. It feeds the first 7 flawlessly. The last one doesn't hit the tongue and pops out when I finish the pump and close the receiver/breech door. The shell usually flops out and hits me in the foot or hits the floor.
Any tips on how to fix this? I'm tempted to try to stretch the spring or to look for an aftermerket spring that has a little more guts to it.