I just sold a 2013 335DS that we had sitting next to the house plugged in. Never had an issue with the steps, while it was plugged in, but the buyer stopped at a rest area on the way home and the steps wouldn't open.
Are there separate circuits for power under battery operation than when it is plugged into shore power?
A few years ago I had an issue where they wouldn't extend after it was unplugged and in storage. After starting the engine, a minute later the stairs extended. The batteries have been checked when I had this issue, and the service said they were OK.