@beastlymm
I should state I have had this happen before the Christmas event.
A Christmas Daily Challenge would stop some of my characters being assigned, and if I rejected it the characters they would still not be assigned.
Even though the task was no longer at the top of their list and it was not highlighted yellow.
Result. ~
Going to Other Springfield and back reseted this and then they could be assigned as normal from the Office Of Unemployment.
It happened with a few characters but also with Channel 6 Broadcast's and growing crops at Cletus's Farm.
Something in the Christmas update caused it to happen more often, is it possible there is a remaining glitch of this on your device.
Christmas Tree Ralph will now be assigned on 24hr task, whatever the problem was, it is now gone.