Do you have any way of getting a picture of your grip? I tried what you're describing and can't make it happen unless I don't arch my right thumb up. My grip has the joint of my right thumb making contact at the top of the left grip panel right behind the slide release, but I arch the end of my thumb well up over the slide release - it isn't doing anything but hanging out in midair just ahead of the decocker. My left thumb is well forward of the slide release with the area just behind the nail riding under the takedown lever. So the slide release has open air around it. My thumb joints are pretty big, though, and that helps keep the tips of my thumbs away from stuff.
ETA that I just tried this again, and I can actually only make this happen with my right thumb in a strong-hand-only grip. A two-handed grip has my left hand too high to let my right thumb get under the slide release.
Do you mean that your left hand thumb meat is doing this? Do you have the heel of your left hand all the way back in the empty area of the left side of the grip? I just saw that I can kind of make this happen if I advance my left hand further forward than usual, but that leaves a hole exposed on the left side grip panel. A person would have to have pretty good-sized hands to cover that hole with their heel and also still force the slide release up with the meat of their left thumb, but it looks possible. Just not for me.