I would first be inclined to buy a Wilson mag, such as a 47D model. There are other good mags to be sure but, I trust Wilson's and use them for all my 1911 stuff.
As for the slide releasing when you "drop" the mag...do you have to physically remove the mag from the pistol? As in, does it seem stuck until you use your fingers to pull it out? When the mag does release...the slide then instantly goes forward?
Take a look at what Jim suggested...remove the slide as if you were disassembling the pistol for cleaning. Then reinsert the slide stop and insert an unloaded mag into the mag well. You should then be able to see the way the mag follower pushes up on the slide stop...or not. It might also be snagging somehow on the slide stop and when the mag is removed, it pulls down on the slide stop, which would of course let the slide move forward.
Let us know what you see.
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