Yeah, it's the firmware. It's an AM4 board, and the BIOS only gives PCI devices the space from the top of RAM up to 512 GB. Each 32 GB BAR has to start on a 32 GB boundary, and its small doorbell BAR spills into the next slot, so each card needs 64 GB. Six won't fit. So I turn Resizable BAR off in the BIOS and resize the four R9700s from Linux, where I hit the kernel bug.