Only from the start to the 'bootconsole [early0] disabled' lines I got this:
That suggests that the issue is with U-Boot's configuration of the UART, rather than the configuration Linux eventually establishes when it starts using the serial port in the way it will when fully booted.
As I've said before, slight innacuracy serial baud rate may be the issue, especially if it shows up with some things on the other end of the serial cable and not others. There are various ways of configuring baud rate generation in the MT7688 UART, some of which will give more accurate results than others.
Other suspects include loose connections, or power situations where some of the supply return current is flowing through the serial connection's ground, introducing a voltage drop there and thus difference in effective ground levels. The observed early vs. ultimate console difference would seemingly point away from that possibility, but then power consumption is different at different points during boot.