I too had the ‘disarmed’ bug when SmartThings integration would randomly change the Arlo Base Station to ‘disarmed’. However, I found a workaround that seems to make Arlo stay in the mode I select on the Arlo website. What I did was log into the Arlo website and changed the Base Station from ‘disarmed’ which the SmartThings integration randomly sets it to, to ‘Armed’ mode. I then power cycled my Arlo Base Station. I guess I could have just rebooted it, but power cycling it was what I did. Anyway, now the Arlo Base Station stays in ‘Armed’ mode and the ‘disarmed’ bug is gone. That is until I reset or change the Arlo connected service again in SmartThings then I’ll have to do it all over again.
Update: I am now able to use the Arlo app to set the base station armed mode instead of the Arlo website before I power cycle the base station to get it to stick.