Hi,
My cheap CH340 based USB-TTL serial cable causes same issue.
The thread of https://forum.radxa.com/t/u-boot-cant-boot-with-serial-console-attached/7684 is reporting this kind of issue.
In my case, FT232RL, PL2303TA based USB-TTL serial cable were stable.
Anyway, it seems that your Rock 3A is not dead but has a trouble, good luck!