I took a look at the mags and the grips tonight.
The MecGar mags have a different curve radious on the mag's nose than the CZ mags. It starts just above the mag release hole. The taper is similar, but the radius of the curve is very noticeably bigger on the CZ mags.
If you look at the wear marks made by the mag release, you'll see rub marks all the way to the very top of the mag with the CZs. The wear marks stop at the mag release hole with the MecGars because the mag's curve is a smaller radius starting at the mag retention/release hole.
The result is if you press the release button fully in, the button end of the bar will contact the nose of the mag. If you press the release button really fast and let go, the back side of the release/retention bar catches on the mags tapered nose before the mag the mag "nose" has dropped below the release. I f you press juuuust the right distance in and don't release the button until the tapered portion of the mag drops below the release bar, it'll drop fast and smoothly.
The MecGars curved portion of the mag's front end (nose) above the mag release/retention bar never contacts the release bar..
They drop with a fast push on the release.
For someone with the thumb reach to press the button using the strong hand thumb without breaking their grip, the button could be pushed just right or pushed a second time and it will release while the weak hand is reaching for the new mag.
Those that that have to use the weak hand thumb to jab the release on the way to get a new mag, the CZ 18 rd mags frequently don't drop.
I don't see an easy fix.
Checked my grips and they clear the mag by a good margin.
Hope my poor typing on the cell phone made sense.