No, that is not what is happening. If that was what happened, then I could get the tile back by clicking once to get the tile back to that city, and again to force a citizen into the tile. What happens when I force a citizen to the farm next south-east of the city is, instead of getting the lock icon, I get the Roman coin head. If I exit the city screen and come back, the citizen is reallocated by the governor to one of the hills.
I "solved" the problem by resetting my citizens, and it let me lock that tile, but that forced unlock of the truffles tile. I can replicate this, and the truffle is not in range of Sale. Doing that, and changing the governor to science focus lets me get the tiles I want, but this is not normal behavior of any kind.