That is very unusual, as you are flashing LEDE's own stable firmware image, with nothing extra in between.
The only possible thing I could think of—notwithstanding a possible hardware defect—is the factory firmware image is doing something when switching over to LEDE. Curious to see if anyone else has had success or failure when flashing LEDE from NetGear's stock firmware version 1.0.2.44. If it's feasible, see if the flash works on previous firmwares (e.g. I used stock version 1.0.2.36 to flash to LEDE).
I've had one bootloop happen to me as well when going from LEDE back to stock firmware via a forced .img flash. I had to fix it by instead flashing the stock firmware over TFTP mode.
That all being said, I cannot think of any plausible cause as to why your R7800 doesn't appear to get along with LEDE. I would recommend you give hnyman's builds a try—his stable or master builds both work very well on the R7800.
As a last resort, you could try what is known as a "clean flash". This usually entails first resetting the current working firmware to defaults (though if your desire is to flash a different FW, you can skip this step and the next), flashing the same firmware on itself (first flash of factory firmware), then flashing the new, desired firmware and resetting once more (new firmware flash + a reset). LEDE has a nice feature that lets you decide to keep settings or to erase them after flashing (only works with OpenWRT/LEDE firmware though, to my knowledge). So in the case of going from LEDE to LEDE (different firmware versions though), a clean flash is largely unnecessary.