I have an older  preban Oly forged  lower (SGW marked / 83 vintage)) mated to an Oly upper of the same vintage.  This was recently rebuilt by an excellent  gunsmith into a M4 configuration using Bushy barrel and collapsable stock/buffer  from a CAR15 configuration.  I've since discovered that when I release the bolt from locked to the rear on some empty magazines (colt 20s) that the magazine is also getting released when the bolt slams closed.  It doesn't do it on all mags and the mags in question do not  malfunction during bolt release  in my Colt AR15. The magazine fit in the Oly has never seen as positive as in the Colt (tolerences I guess).   I didn't have this problem before the rebuild, any suggestions?

You may want to tighten the mag catch one more turn.  
In general, the mag catch threaded post tip should be flush with the surface of the mag catch button.

To tighten the mag catch, use a pencil and press the mag catch button all the way in until the button bottoms out, then reach around the other side of the receiver add spin the catch one more turn (or needed) clockwise.