It has an alloy follower, body looks fine, and it has a brand new USGI spring in it.

It functions great except on the 19th round I'll pull the trigger it will fire the bolt will lock back and the last round will just be laying on top of the mag sometimes part way in the battery some how. But I can tilt the rifle and it will fall right out. Or I can press the bolt release and it will chamber normally and shoot fine.

Any ideas?

I've heard/read somewhere that if the spring was installed backwards under the follower, upon reassembly malfunctions could occur, which led to the "load only 18 rounds" in a 20 round mag practice.  Maybe somebody else can verify?

Could be that the mag lips are spread out or worn too much to hold the last round in, so that when the bolt comes back after firing the second to last round, the last round pops out, allowing the follower to move up and engage the bolt stop.  

Another possibility is that the follower is canted to one side so that the ridge on the follower doesn't properly hold the last round in the mag.  With the mag empty, see if the follower is seated level in the top of the mag, if not, it could be that the spring is installed wrong, or that it is bent wrong at the top.  Either case, remove spring and install correctly or bend it to get it to hold the follower level.

Put one round in the mag and bend the lip to conform to the case.  Put the second round in and do the same to the other lip.

I've used the same 20 round mag for over 10k rounds.  I've had to adjust the lips only once.