Keyboard unresponsive after some idle time in uboot

[Trinatriumcitrat]

When I leave the uboot cmdline or grub cmdline open for a while, system or at least the keyboard becomes unresponsive.
It does not happen after Linux has started.
Happens on 2 different Uboot builds, the one from Towboot and the one from Fedora.
The type of bootloader does not matter, it happens in grub/syslinux too.

Can you help me investigate? Is this a known thing?

[KC9UDX]

Are you sure the SOC isn't overheating? I don't think the CPUs are throttled at any time before the operating system is running.