So Linux 6.1 vendor kernel, okay.
HS200 and HS400 are protocol modes to access the eMMC, allowing 200 MB/s and 400 MB/s respectively.
Looks like this fixed the issue: arm64: dts: rock-5a: pull down data-strobe to fix emmc compatibility · armbian/linux-rockchip@7628960 · GitHub
I’ll build a new kernel and image.
… ah wait, looks like the kernel you use has this included already 
.
Maybe indeed enabling HS400ES caused the issue: Enable HS400ES on Rock 5A · armbian/linux-rockchip@1447ae0 · GitHub
Regardless, probably it was fixed in another way. I’ll generate an image with fresh kernel first to test. If this does not work, I’ll generate one with HS400ES disabled, reverting the linked patch.
Same was done on ROCK 5B. Need to keep this in mind in case other report issues booting from eMMC on ROCK 5B as well: Enable HS400ES on Rock 5B · armbian/linux-rockchip@6ccac54 · GitHub