Using Toys to change your character’s model (and probably other methods, although Druid shapeshifting seems to work fine) or mounting/dismounting when these transformations are active, cancels the buffs from several “totem/banner” effects, such as the Highmountain Totem items sold by Slyhoof, or the Necrolord Warrior’s Raise Banner (which is already cancelled all the time by using Conqueror’s Banner)
This is definitely something changed in 9.1, and since there seems to be no benefit to it, it is likely a bug, not an intended change.