The bug
Parrots don't imitate the new mobs in 1.14 (pillager, ravager and panda). The pillagers and the ravagers are aggressive and the pandas are neutral (except worried panda).
MC-138078
Parrots don't imitate pillagers, ravagers and pandas
Resolved
Fixed
11
Confirmed
imitation, panda, parrot, pillager, ravager
Minecraft 18w43b, Minecraft 18w43c, Minecraft 18w44a, Minecraft 18w45a, Minecraft 18w46a, ..., Minecraft 19w05a, Minecraft 19w06a, Minecraft 19w07a, Minecraft 19w08b, Minecraft 19w09a
Minecraft 19w11a
Retrieved
Not ALL Pandas are neutral; only the aggressive ones attack upon being hit. Thus, Pandas not being imitated might be WAI, as parrots also do not mimic passive mobs (i.e. Sheep, Cows, Pigs, etc.)