Running with two DIMMs per channel is more challenging for the CPU's memory controller than one DIMM per channel, and it's much worse if they're dual-rank DIMMs—which current 32GB DDR5 modules are. I believe the EXPO/XMP profiles are all based on using only two DIMMs total, because those modules are sold in kits of two. At some point in the next year, there should be 24GB single-rank modules hitting the market, allowing for 48GB at top speed or 96GB with less of a performance penalty than 128GB from 4x32GB currently causes.