Put a fresh mag in the rifle with the bolt latched to the rear, ride the bolt forward (as you watch through the ejection port) to see how it interacts with the empty magazine. Try a couple times normal, then try to recreate the issue by pushing rifle down against firm surface as you ride the bolt forward
I would assume it is related to a mag height issue. Looks like you are using surpus mags. So it may or may not be a magazine issue..
For the mags, have you been able to reassemble them afterwards? I would drop the floor plate pull the spring the rest of way out the magazine through the top, then reinstall. Check the feedlips carefully for bending or damaged. Also check the magazine for condition and how the feed guide is retained in the magazine.It is unusual for a bolt to catch the feedguide without the feedlips being out of spec. As the bolt rides forward the lower lugs travel between the feedlips to catch the round in the magazine. Unless that portion of the bolt head is out of spec or the magazines are out of spec? I find it odd, first time heard of a G3 clone doing that trick.
I would then reach out to PTR Customer service once you have a solid bead on what is happening. They are usually pretty good with their customer service.