In my experience, the decoders I have bought/installed came with the analog mode (straight DC) feature turned on. I have always had to do something to turn them off.
The decoder waits for a signal and if it does not detect a signal on the rails, it assumes straight DC and uses whatever voltage it sees.
If the analog mode is turned off, set CV 29 to 36 (28 speed steps, analog on).
The other settings possible for Analog on are 6, 22, 38, 52 and 54. However, 52 and 54 apply to programmable speed tables.