I had/have this exact same situation (except my Strips reports 254 - are you sure it was 255?) not only with this sensor, but with Fibaro FGK-101 as well. I got an answer from Sensative - thread also contains more information about when the value changes in my case.
Note that if you are using access_control, using the logic if 22 → open, else closed the “closed” rule also covers value 255 so the door might be open and HA shows it as closed. Same thing if you restart HA, you can’t know the door state before it is first opened/closed.