Alright this is a brand new FNX .45, that has never had a shot fired through it, I bought it back in the summer.  What happens is this:  With the slide locked back, and an empty magazine inserted, if I let the slide go forward into battery the slide release tab will jump over the tab on the magazine follower and prevent the magazine from dropping out when the mag release is pressed.  Is this common?  I just ordered a couple of extra magazines to see if its just my mags, or if it does it with all of them.

First I've heard of it

Not sure how this is possible.  On my FNX45T, my slide release tab has a 90deg extension that sits over the magazine follower by about 1/8th inch.  If this tab is short, you might get that malfunction but that would be on FNH to fix or just buy a new slide release.

Yes mine is the same way, what I think happens on the empty magazine, is that as the slide goes forward it forces the magazine follower down and to the right just enough for the slide catch to slip past it.  Once it does that of course it is hooked under the follower and will not let the magazine drop free until I remove the slide and force the follower over enough for the catch to pop back up.

I have one magazine that does exactly what you're describing. CIB pretty much nailed it, it's a follower issue.

This happens with my M&P Core. It's a follower issue on that from what I read, and is more prone when the mag release is switched for lefties.

Just a quick update, I received my new FNX mags from PSA's sale on them.  Out of the 5 that I now have, only one of them will allow the slide catch to slip under the shelf on the magazine.  So from that I deduce that probably one of the follower molds is a touch out of whack, and causes this malfunction with some of the magazines.