It’s difficult to say for sure with certainty what the issue is without trial and error. I would expect that the motherboard’s manufacturer would make sure that their board can successfully pass all tests with the standard JEDEC spec for DDR4 (2133 MHz).
Since you say that you’ve tried different RAM kits, another alternative could be the cleanliness of power from the power supply. Perhaps there is intermittent voltage droop, and you need to experiment with the Load Line Calibration settings to adjust for vdroop between idle and load. Disabling frequency boosting and manually setting the CPU frequency could help check if it’s related to that. PBO curves might be undervolting too much while idle.