KM01 Curtain module doesn't response to switch

I recently received a KM01 for controlling my sunscreen.

When I set it up without an extra switch, like below, I can control the sunscreen from the webUI just fine. Calibration went well and when I command it to 50%, the screen moves to the right position for 50%.

But, I like to keep the wall mounted switch, so the screen can always be controlled, without having to log in to Domoticz, so I followed this schematic:

Simple enought. Jet, when connected this way, the switch isn't working, no matter how I config it in Tasmota (following the instructions on the site of Tasmota). When I set the switchmode of both switches to mode 15 (so I can see detection of the switch in the console), nothing happens when I operate the switch. I can still operate the sunscreen from the webUI though.

However, and this is the strange thing, when I connect S1 and S2 in parallel with the wall switch, with the sunscreen motor still connected to the wall switch (so I can't control the screen from the webUI of the KM01, just detect switch input), I do get a response in the console when I operate the swich:

console-output:

20:44:54.805 CMD: SetOption114 1
20:44:54.811 MQT: stat/tasmota_DDD17D/RESULT = {"SetOption114":"ON"}
20:45:12.954 CMD: SwitchMode1 15
20:45:12.961 MQT: stat/tasmota_DDD17D/RESULT = {"SwitchMode1":15}
20:45:15.024 CMD: SwitchMode2 15
20:45:15.030 MQT: stat/tasmota_DDD17D/RESULT = {"SwitchMode2":15}
20:46:25.121 MQT: tele/tasmota_DDD17D/SENSOR = {"Time":"2023-06-25T20:46:25","Switch1":"OFF","Switch2":"OFF","Shutter1":{"Position":0,"Direction":0,"Target":0,"Tilt":0}}
20:46:25.421 MQT: tele/tasmota_DDD17D/SENSOR = {"Time":"2023-06-25T20:46:25","Switch1":"ON","Switch2":"OFF","Shutter1":{"Position":0,"Direction":0,"Target":0,"Tilt":0}}
20:46:26.324 MQT: tele/tasmota_DDD17D/SENSOR = {"Time":"2023-06-25T20:46:26","Switch1":"OFF","Switch2":"OFF","Shutter1":{"Position":0,"Direction":0,"Target":0,"Tilt":0}}
20:46:27.226 MQT: tele/tasmota_DDD17D/SENSOR = {"Time":"2023-06-25T20:46:27","Switch1":"OFF","Switch2":"ON","Shutter1":{"Position":0,"Direction":0,"Target":0,"Tilt":0}}

When I disconnect the motor from the switch, the KM01 stops responding again.

I've tried different SwitchDebounce settings (300 and 69, as advised by Tasmota), but to no avail...

To be clear, the wall switch switches live 230V between both outputs, no matter if het motor is connected or not. So in theory the motor shouldn't make any difference, but somehow it does when it's parallel to the S-inputs...

I'm at a lost... Who can tell me what's going wrong?