Mine has the Ambi Mag Drop & I find that using the right side instead of the "normal" left side works better. I don't use it for an EDC, so I haven't had the Mag Release swapped out for the non-Ambi.
If that isn't your issue, please try to be more specific...
I have never seen an issue with the G21SF and magazines dropping free. Evansville PD has 300 of these weapons and I have ran qualifications multiple times using these weapons. They are just as reliable as the G21s that they replaced.
We had a few standard G21's that had broken frame rails in the rear. Even with the broken rails the guns continued to run 100% and the issue was discovered at armorers inspection. If I recall these were all SWAT guns with high round counts. Glock replaced them without issue. This was a known issue with E series guns, but ours were DVP prefix guns. My DVP G21 had probably 25k through it with no issues. My current G21SF is well over 2500rds with no issues.
That was one of the reasons the Indiana State Police are moving away from Glock 21s. The drop free mag issue is a big one with them and too many G21s failed in that respect. I don't own a G21, but I do have a G29 that does not drop mags free. I also own and carry CZ 75s, so it does not bother me too much.