Reckon I’m going to throw my 2cent in. With it being rebuilt in the last couple years, my gut says it wasn’t torqued properly. Because all mechanics have tricks, I won’t criticize what may have worked for others, but I def would not attempt to re-torque, unless this was within a few days of installation. Reason is, once the gasket leaks, any foreign matter that is between the gasket and surface, could burr or gouge the machined surface.
I totally agree with the gasket replacement on the head. Since it was rebuilt earlier, I don’t think your rings or cylinders need repair, but as suggested even , I would check piston tolerances. You can get an inexpensive micrometer at your local parts house. You will need a ring compressor, because once you remove the heads, about the only way to put it back on without, is if you got the engine on a workbench.
Being honest, I’d be pissed, after spending money on an engine that is supposed to last between 200-300 hours, and this occurs. Personally, I’d be banging down the door of the shop that did the job, and if no solution, social media would become a close friend. That should NOT have happened under normal riding conditions!
Just my 2 cents.