Not sure what it could be, but maybe a hint for fault finding. Does it do it both with the automatic function when you place it on a table, and when the manuel move on the setup menu is used? If it is only in automatic mode it could maybe be related to the activation switches.   On another note belt size and tension can  be very important. I was advised about the belt size in this post for reference:

Location: Denmark

I have since come to a better understanding of how the "Auto" function works. There are two buttons on the bottom disguised as feet. When you set the unit down, the button contact closes and the unit goes in the the "raised" position. When you lift the unit the contact opens, setting it to the "down" state. It is no more complicated than: Closed=Raised, Open=Down. In my case my battery cover was bulging due to temporary wiring, so the button could only close briefly when setting it down. A temporary lift due to temporary button press. Problem solved!

Glad to hear you figured it out. Testing stuff that is not fully assembled can sometimes introduce veird anomolies