I would attribute this to a weak hold open spring. Due to the increased recoil, caused by the lowered weight of gun + mag + 2 remaining rounds, the hold open jumps up and blocks the toggle prematurely.
Normally the hold open is pushed upwards by the magazine button after the last round. With one round still in the mag, the magazine button is too far down to be able to work the hold-open so I'm pretty sure it is not a magazine issue.
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