The electrical in-rush that the Tunze ATO sees when you plug it in will be the same if via AC plug or DC plug, so I'm not sure this has caused a problem with your device (I can't guarantee it is not, however, as electronics can fail in many ways).
What I can confirm is a similar experience I had last week when my Tunze ATO started beeping by itself in a power cycle loop. I was able to make the Tunze reboot by simply wiggling or twisting the power cable, and sometimes it would not power on at all if the angle was right.
After closer inspection there was a very small amount of buildup (possibly corrosion) on the contact inside the Tunze (not the pin, but the side pressure contact). If you look, you might be surprised at how little metalic material makes contact between the plug and the receptacle.
To fix, I took an exacto knife and gently scraped/scratched/rubbed the middle of the contact (the sub-millimeter area) where the electrical contact actually happens... and presto all is fixed, and I can't force it to reboot anymore.
Cleaning this contact is now in my monthly maintenance procedure as I don't want my ATO to fail
Hope this helps (you or someone else -- it sure helped me).