In WP7702 the UART1 driver have the power management independent of system suspend/resume.
UART1 wakeup can be triggered by RX or TX activity. But before it wakes up from RX, it loses several bits from the RX and then got out of sync.
The default auto suspend delay is 5 second.
root@swi-mdm9x28-wp:~# cat /sys/devices/platform/msm_serial_hs.0/power/autosuspend_delay_ms
5000
There is a possiblility to increase this auto suspend by changing this value.
E.g. change the suspend delay to 20 sec:
echo 20000 > /sys/devices/platform/msm_serial_hs.0/power/autosuspend_delay_ms
Maybe there is a possiblility to disable the suspend completely.