Octatrack battery question

I’ve had my OT mk1 for about 4 years. I leave the power plug in all of the time but turn the powerstrip it and all my other gear is connected to off when not in use. One thing I noticed is that when I disconnect the power plug (like when taking it to a gig) for a short amount of time I lose the date and last used set.

I ran the self-diagnostics and it looks like the battery is okay. Also I can leave my rig off for days or weeks and this does not happen. Only when it’s stored with the power plugged disconnected.

Is this normal?

In case if you want to check / change the battery :