The initialization procedure seems to be a problem.
The TXD signal and Port 71 share the same pin.
After a reset operation, this pin operates as an input port.
If PM71 is set to output here, the contents of the output latch will be output.
After the reset operation, the output latch is 0, and if it remains as an output port, a low level (the same level as the start bit) will be output.
This status continues until the output latch is set to 1.
If this period is longer than the time of 1/2 a bit determined by the baud rate, this state is interpreted as a start bit and results in the aforementioned problem.
Initialization should be performed in the following order: set the output latch of P71 to 1 and then set PM71 to output port mode.