JDY-16 slave suddenly switches to echo mode

Hello,

I connect two JDY-16 modules, firmware version 2.3, one as master, the 
other one as slave.

To configure the slave, I only change baudrate to 115200 with

AT+BAUD8 and a reset
AT+RST

To configure the master, I use the following commands:

AT+HOSTEN1
AT+SCAN (to find out the slaves MAC)
then I pair both with
AT+BANDxx.. , where xx.. is the MAC address

This way both are connected as soon as both get power.

It works perfectly, the master sends every second a command and gets the 
answer from the slave, but after some hours the slave starts to echo the 
commands. This behavour remains after a power cycle. With factory reset 
I was able to go back to the echo-less state, but not for long and the 
same thing started over.

I found out, that the modules TX line pulls down the RX line internally 
when this happens. Apparently the internal pull-down resistor is about 
250 ohms.

So I found a work-around by pulling RX to 3.3 V with a 100-ohm resistor.
This works but maybe anyone of you has an idea, how the module can get 
into this echo state.

According to the few infos I have about this module, there is'nt even a 
command for echo mode.

Thank you

Rudi