WIFI 1010 not running code at power-up while RX is connected

Hi,

I am using MKR WIFI 1010 as a main controller connected via TX/RX pins to Arduino Pro Mini acting as a data collector.

Everything works well except for power up. When MKR starts it acts as if I would double tap the reset button. At this time I can re-flash it and everything works well, but is is not starting flashed software, not until I restart it by using the reset button.

I disconnected the Pro Mini and found that the strange behavior occurs only when the MKR RX pin is connected to the Pro Mini TX pin. Without this connection, MKR starts normally and run flashed code upon power cycle.

Ideas?

[EDITED]
Problematic pin is TX, not RX as I wrote before, details below.

is the Mini 3.3 V?

The MKR RX pin is the one causing problems (and not TX as stated before).

Adding a 1K resistor between the MKR RX and the Mini TX seems to have solved the problem.

Does this makes any sense? Why would the MRK go into bootloader mode if it senses a signal (high, low, i don't know) on GPIO13/RX pin?

Juraj:
is the Mini 3.3 V?

Yes, ATmega328P.

Mini Pro can be 5V or 3.3 V version. is yours a 3.3V version?

Juraj, I already answered Yes, 3.3V.