I guess some features will only work when they are allowed at startup/boot. Pin at boot or full encryption prevents everything from working at boot or startup. You can still enable the pin/pattern at startup after enabling Dynamic Contrast if you want and the latter will still. However, if by any chance that your device restarts, Dynamic Contrast will stop working again (but you can still repeat the process though).
I guess the Direct Boot mode of Nougat/Android 7.0 will fix this bug.