Have you tried different mags? I have seen this problem in a few guns, and it was related to the insertion depth of the mag. If it doesn't sit deep enough, it won't push up the hold open far enough to catch the bolt. I have fixed a few by simply swapping mags and hold opens (could possibly be related to the mag release as well), but you may not want to do that if the gun is matching.
You could take the mag out, then work the latch with a screwdriver to see if it catches the bolt. It will probably be a three hand operation, but it should be possible. If it does catch, I'd say the mag is just not pushing it far enough.
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