It sounds like you have a slower DIMM slotted higher in the order, so you’re artificially limiting the faster DIMMs ranked behind it.
Look up the optimal slot order and speed requirements etc for the motherboard, I’ll bet you’ll find you have it stacked wrong or have technically unsupported DIMMs.
You're right on both counts, I think. I'll have to double check my pair order.
This is a hackintosh so it's a bit of a mess. The pairs are mismatched in speed, but if I turn on XMP to get it up to the speed of the lower pair (which is supported by the higher) then I get occasional kernel panics in macOS.