this is the code i am using from the gpy to the esp
he sends a "?" when the esp receives the "?" the esp sends the data to the gpy

from machine import UART
import time
import pycom


pycom.heartbeat(False)

def read_esp():
  uart = UART(1, baudrate=9600, pins=('P21','P20')) # voor esp

  strMsg = ''
 
  uart.write('?')

  time.sleep(2)

  if uart.any() > 0:
    
      strMsg = uart.read() 
      print(strMsg)
   
      strMsg = str(strMsg, 'utf-8')
      print(strMsg)
      return strMsg 
  else:
    strMsg = "noData"
    print(strMsg)
    return strMsg 

read_esp()

This is the code from the esp

import machine
uart = machine.UART(2, 9600, txbuf=512, rxbuf=512)

strMsg = ''

while True:
    if uart.any() > 0:
        strMsg = uart.read()
        print(strMsg)
        
        if '?' in strMsg:
            uart.write('033,3.3,+22,10,1,10')
        else:
            uart.write('error')
            ```

i use the expansionboard 3.0 to communicate with an esp via uart.
i have a sensor on the standard uart pins of the gpy p3 and p4 and in addition i want to communicate via uart on pins 20 and 21 with an esp
In the picture you can see my setup.
When i power the gpy and the esp both through the usb (gnd and 3.3v not connected to esp) then the uart works. When i power the gpy from battery only, (gnd and 3.3v connected to esp) then the uart does not work. I don't know why.

this is the code i am using from the gpy to the esp
he sends a "?" when the esp receives the "?" the esp sends the data to the gpy

from machine import UART
import time
import pycom


pycom.heartbeat(False)

def read_esp():
  uart = UART(1, baudrate=9600, pins=('P21','P20')) # voor esp

  strMsg = ''
 
  uart.write('?')

  time.sleep(2)

  if uart.any() > 0:
    
      strMsg = uart.read() 
      print(strMsg)
   
      strMsg = str(strMsg, 'utf-8')
      print(strMsg)
      return strMsg 
  else:
    strMsg = "noData"
    print(strMsg)
    return strMsg 

read_esp()

This is the code from the esp

import machine
uart = machine.UART(2, 9600, txbuf=512, rxbuf=512)

strMsg = ''

while True:
    if uart.any() > 0:
        strMsg = uart.read()
        print(strMsg)
        
        if '?' in strMsg:
            uart.write('033,3.3,+22,10,1,10')
        else:
            uart.write('error')
            

One thing you could try is connect RX to TX on either side to see if you still receive what you send (on both sides). This could help to isolate which side is the problem.

I am moving UART1 to pins P20 and P21

uart = UART(1, baudrate=9600, pins=('P21','P20'))

If you are using sleep modes and the like, the firmware may talk to the modem and reconfigure UART1 without you explicitly doing anything to it. Likewise with Pybytes.