What can trigger this unwanted start signal?

Hi there electro geniouses!

I have a semi-automatic labelling machine LT-50D that is malfunctioning in a strange way. Here's how the machine should work: https://www.youtube.com/watch?v=FmlPliTG...

I.e. Pull handle down manually => microswitch closes circuit => start signal to the motor => 1 label rolls on and the label roll stops where it should when a sensor detects the label end and sends a stop signal.

But what happens in addition to this is:

Handle lifted up => microswitch opens the circuit => another start signal to the motor => 1 more label rolls onto the same bottle (and then the label roll stops where it should).

The big question is: What condition or component could cause this behaviour?

I tried replacing the microswitch (vendor's only suggestion so far) and also to remove the microswitch and close the circuit by hand with the wires, and the result is the same. So it's not just a faulty switch.

The malfunction was not present when the machine was brand new but started appearing after a short while. Sometimes when the machine was just turned on it would function mostly correctly for fifteen minutes (malfunction only 1 in 15 labels) but then 'degrade' to every time. And now it's every time.

Do you think it would solve the problem if I send the circuitry board back to the vendor and get it replaced?