Just discovered a bug while playing on a survival realm server.

Was on horseback at time, saw a wolf to tame. Successfully tamed the wolf whilst still on horseback and once tamed, wolf immediately started to attack the horse like they do when in aggression mode. I jumped off but wolf continued to attack horse and stopped once it died.

Have since tested this in creative and same outcome.

Pretty easy to replicate in creative by taming horse then riding and taming a wolf at same time.