For those who went out of their way for the quest:
Shiny Alolan Vulpix cannot be encountered through Field Research
Issue description: A bug is causing Shiny Alolan Vulpix to not be encounterable through Field Research where an Alolan Vulpix encounter is the reward. This bug only affects Field Research encounters, and there is still a chance to hatch Shiny Alolan Vulpix via Eggs.
Issue status: Investigating
Last updated: January 8th