Boots okay, but "Card did not respond to voltage select!"

Booting FreeBSD from an external micro-SD (mmc0) card has no issues.

The same image booted off the eMMC (mmc1) causes a flood of messages:
“Card did not respond to voltage select!”

However, everything works fine. I believe, the message comes from the MMC driver of U-Boot (mmc.c).
What can be wrong?

Both mmc0 and mmc1 operate at 50MHz.

Thanks for advises!