I'm probably telling you something you already know, but you might want to take a look at what you are sealing the threads with. While messy, PFTE paste is way better than the PFTE tape or no sealant at all. Would also take a look at how good the seal is between the hose and plug as well as the seal between the plug and the tube.
I had to perform hydro-testing of welded components for fire equipment a few years back and ended up using expanding plugs in the ID. Some are mechanical and some are air pressure operated. All are available with a pass through fitting for gas or air pressure. The only thing that can be problematic with this approach is if the tube has an internal longitudinal welded seam. They are also available that seal on the OD of the tube. They are designed for production use and have replaceable sealing components.
Careful inspection of rubber seals each use is key as they do wear, tear and cut.
Just so I cover all the bases, I assume by now you have looked at the mill reports on the tubing you are using now and before and found them in spec, so I was wondering if there is a difference in humidity between where the tubing is being stored and where it is being welded. Thinking about condensation inside the tube, even from something weird like morning dew, overnight to daytime temps, outside storage to inside welding etc...
Hope some of this rambling helps!