Well, I've done some sleuthing. The magazines drop freely when empty. They only hang up when loaded.
Upon close inspection, it appears the magazine safety bar (yes, mine has the mag safety) is pressing so hard on the back of the magazine that it is preventing it from dropping. With the slide removed, and loaded magazine inserted, I can actually see the tip of the mag safety bar engage the primer pocket on the next cartridge and lock up against it. When pushing the release the mag drops 1/4 of an in. or so until the tip of the mag safety bar finds the rear of the cartridge and primer pocket.
With a small dental pick, if I gently pull back on the mag safety bar, the mag immediately drops. The spring tension on the mag safety is quite stout, so it is pressed very solidly against the back of the magazine and/or cartridge.
I will either need to modify the spring or the mag safety bar. I'm thinking that some very judicious filing (more like polishing) the leading edge of the safety bar may shorten it by say a few thousandths(?) so that it cannot either: 1. press so hard against the magazine, or 2, have the tip engage the primer pocket on the cartridge.
Any thoughts?
ETA: I've also just noticed that the back of the magazines shows a wear pattern with a groove started exactly where the mag safety bar is wearing against the magazine. I may try to alter one of the mags by polishing that spot to see if I can even out this wear mark.
ETA2: Polishing the mag did nothing. The tip of the mag safety bar seems to be the culprit, catching on the primer.