I’ve made a Python script using the Python library for it that reads the input #1 twice per second to detect a button press (The button has one wire connected to the ground, the other to the input #1, the jumper is set to UP).
When I run that script, I see the value “255” until I press the button, then I see “0”. This seemed to be what is expected and it works reliably.
I sent that script to someone else who has the same relay board, he has a different experience though, he sees the value “255”, but when he presses the button, the library returns “False”, so I dug in that library and I found this:
When he sees “255”, the library has this data:
read_data: b'\xaa\x01\xff\xaa' handshake: True bytes_back: True checksum: True 
When he sees “False”, the library has this data:
read_data: b'\xff\xff\xff\xaa' handshake: False bytes_back: False checksum: False False
We also noticed that we have more chance of success if we use the ground on the right instead of the ground on the left for the button.
Voltage reads: 4.90v
The same code works correctly on 2 relay boards and it doesn’t work on 2 other different boards…
Anyone has an idea?