I suggest that you submit a support ticket at:
support.opensprinkler.com
with some basic information, and send your controller back to us so we can check and reflash the EEPROM for you so that it will recognize it as DC again. Without it being recognized as DC, the voltage booster circuit won’t engage and therefore it won’t be able to energize the valves with low-voltage power adapter.
But you are right that you can actually use 12VDC to power the controller and if it is able to energize the valves, then you can continue doing that. Technically 12VDC is a bit low for producing inrush current, while a bit high for providing holding current, but I’ve seen people using it for years without problem. So if this works for you you can continue using it.
The design of DC-powered controller is optimized for providing the right currents for both inrush and holding. It produces a boosted voltage about 21VDC to energize the valves. Given that a typical 24VAC valve has a resistance of about 35ohm, this corresponds to 600mA inrush current, which matches most valve’s electric spec. Then it lowers the voltage to the input voltage (7.5VDC), corresponding to about 200mA holding current, which is typical.