The default quests for a few legendary Pokemon have mistakes that make them unable to be completed:
The Galar Bird quests (Articuno, Zapdos, Moltres) require "form:ashen", "form:strike" etc, but in 1.16 the formatting for that should be "palette:ashen", "palette:strike" etc. This may be true for other quests, I haven't done a thorough search yet.
The Drowned Lugia quest spawns a Lugia with palette:drowned and boss:drowned, but to complete the quest it must have palette:drowned, boss:drowned, [/i]and[/i] ability:revenant. Since the spawned Lugia doesn't have the ability, it's impossible to complete the quest.