I posed this question on another forum, where a member pointed out that the latch is driven by the mainspring, and it presses against the mag. I took the grip panels off to take a look at what is really going on with the release, and sure enough he is right.
The heel-mounted latch does press against the back of the mags, so there is always friction. "How much friction?" you may ask. Enough that I can hold the pistol upside down by the unlatched mag, and it doesn't move. Retentive, indeed!
I had not previously noticed that the latch was driven by the mainspring. Now I am wondering if a spring swap will make both trigger pull and mag removal a bit easier.
I should have looked before I posted, but the internet was closer than my screwdriver set.