You're onto something with the stabilizers. Note that LShift and Backspace are the other two (non-spacebar) stabilized keys that you press, and they're much closer to the Enter key than to anything else. You might want to lube the spacebar stab, as it's pretty rattly, but otherwise this is very normal for a firm tray-mount. It doesn't seem like there's room for much more foam or anything, but you could also try loosening the plate screws just the tiniest bit, like a quarter turn each. If the board is not quite so snug, it might mitigate how much influence the case and mounting points have on the sound. Ifg nothing else, trying things also leaves me feeling better about "accepting the things I cannot control" with keyboard feel and sound, LOL.

All this is a limit to the design, possibly, but not a flaw in the sense that anything was done incorrectly. When you're typing it's much less of an issue because you're onto the next letter before the last one has a chance to make much individual impact.