When you use a DCC Decoder in DC mode the decoder is looking for a SMOOTH DC power supply.
The decoder is doing two things in DC mode:
1. Looking at the signal on the rails and monitoring it for a DCC signal.
2. Reading the polarity of the DC voltage on the rails several time per second to determine direction of the DC voltage.
If you have a decoder with odd behavior in DC mode it is most likely due to dirty track, dirty wheels, and / or poor track pickup. The reason is that intermittent contact with DC will confuse the decoder and result in awkward stops, starts, and random changes in direction. This is not an issue with the decoder but an issue with track pickup that cannot be solved by the decoder per se.
This is also similar to the behavior you will see on a DCC system if no DCC signal is present on the rails.
If you have a DCC only layout the best option is to disable dual mode by modifying CV29.