I think the answer to this, all comes down to the available food supply in the area
if there is a LOT of food they can be more picky on IF they will "X" any thing
when food is less, they ANYTHING they can to get by
and you have to remember deer numbers come into play too
HIGH deer numbers , and less food, deer again eat ANYTHING, low deer numbers and less food, deer will eat what they porefer first then onto different things

add in the fact NOT all brassicia's/rape/turnips(all in the same family of plants here)
there just NOT all the same , and then the fact they tend to be bitter to most critters when growing, and DON"T become appealing till after some frosts(again pending deer nunbers deer on one farm I managed SUPER high deer numbers would eat it as soon as it came out of the ground till all gone)

SO< if there is better food farther away than your plots OR just where deer tend to be in colder times, then you plot is NOT getting hit due maybe the deer just are else where feeding on things they have been known to eat
if you folflow me here
they might just NOT know your plots is NOW good, to eat!
so yes a learning curve might be in the cards, but also, a lot again depends on overall food in the area
IS there something better they like ? Corn and beans are hard for deer to pass up in the colder weather!
and if they stick to that and then the ground freezes, your turnip bulbs and such are again maybe more work to get to than other things?