From what I’m personally seeing, and hearing back from friends.
If Pathfinder was unlocked before Shadowlands [flyable] works on existing characters, but new characters are stuck with it non-functioning.
Seems like the flag is no longer getting set in Shadowlands.
-Another Related Bug Report-
https://us.forums.blizzard.com/en/wow/t/flyable-macro-not-working-in-wod/682310/8