I can lock the bolt open from inside the mag well no issues there.
Are you doing this by inserting finger in and pushing the magazine catch/bolt hold open up while cocking the rifle?
When you do this the bolt hold open goes to maximum up, as it were and the bolt stays open.
Bolt hold open catch, the little bit that the empty follower lifts to hold the bolt to the rear.
(I am at a loss for words for the exact part name and can't seem to chase down my Norinco parts breakdown in bad Chingrish books.)
Magazines in the Gen 1 vary wildly even within the same brands.
Some will not even be enough to lift and lock open on the empty magazine.
Some will push up on the bolt hold open catch "just" enough to lock open when empty but not high enough to keep the bolt locked open when the mag comes out. Try it dry with as many different magazines as you can to see if they differ.
Otherwise take a closer look at the part for either a lot of wear or any signs of tool marks from someone fidgeting with it.
Last resort is to change the bolt hold open.
A lot of those Gen 1s have some hideous wear from everything from guys filing and sanding opne the mag wells because the original PMags were too tight and so forth. Don't let yourself loose your mind to what you see. The important stuff is loads, cycles and ejects.