Quoting myself from another post:
Please don't use MemTest86. It's outdated and not suitable for modern hardware.
Everyone uses HCI MemTest, spawning as many copies as the CPU has threads, evenly splitting the total RAM between instances, and letting it run to 2000%.
If you want something faster, consider Kahru RAM Test. It's not free but picks up on errors extremely fast that sometimes HCI will miss.
As for your setup, two things come to mind: the board and the CPU. The ROG Strix B450-F uses a daisy-chain layout, meaning that tends to prefer only two sticks instead of having all four DIMM slots populated like T-topology.
The other possibility is that your 2700X has a terrible integrated memory controller and can't handle 4x8GB at high speeds. My 2700X couldn't handle 4x8GB of 3200 CL14 no matter what I tried. I swapped the CPU out for a 3700X and it seems completely stable.
What happens if you re-test (with a proper tool) with only two sticks in dual-channel mode with XMP enabled?