Definitely sounds like a mismatched buffer/spring setup. I've had a complete factory LWRC upper in M6A2 and M6A2-S (mid-length gas). Both came with a supplied buffer from LWRC. My buffers were standard carbine weight buffers and not H2...which made me wonder why they even bothered to package them together. Both uppers ran flawlessly with no issues and perfect 3-4 o'clock brass ejection pattern. Still, if your upper ran perfectly with an H2 buffer, you need to run an H2 buffer.
Sounds like the JP buffer you have is an H or H1. If your upper likes an H2 buffer and you try to run a lighter buffer, you can usually get one of two issues. More likely I think you'd get more aggressive cycling, but you would get full cycling. In your case you describe that you're not getting full cycling...no bolt lock back. I can't comment about the performance of the gas upper you tried, as that throws a whole other list of variables into the mix...barrel length, gas length, etc. The other thing your "too light" buffer could cause, but not as likely from your description, might be "too early" an attempt to extract the fired case. However, this should be somewhat evident by the condition of your fired brass.
If your rifle likes an H2 buffer, then you run an H2 buffer...period. I'm assuming that your M6A2 upper is still totally OEM. You don't mention how many rounds have been fired through this upper. Sometimes a new rifle needs a good many rounds fired through it to "smooth-up" the BCG and barrel extension interface. With a factory LWRC this usually isn't the case, as the NiB BCG and manufacturing tolerances are pretty much spot on out of the box. My LWRC was my first exposure to full NiB BCG, and I was quite impressed with how smooth the BCG cycled in a brand new gun. I kind of doubt you're experiencing a break-in issue with that upper.
Edit: Due to another currently running post about LWRC piston rifle buffers, I just discovered my M6A2 buffer that came with the factory upper is unmarked but appears to be H2...kind of odd. I just inspected my M6A2-S buffer, and it is marked H2. Both buffers weigh just over 4.5oz, so they're both clearly H2 buffers. Not sure why the M6A2 buffer wasn't marked since both uppers were brand new factory uppers and had a buffer shipped with them. The unmarked buffer has been etched "H2" to avoid confusion in the future. I'm glad these posts caused me to check and realize the issue. I'm sure this is not common practice for a mis-labeled buffer to make it out to the public, but in the future it will cause me to throw any buffer on the weight scale regardless of markings just to be sure.