There are actually over a dozen war steed cosmetic bugs right now, including the one you mention. There is an invisible caparison, numerous pieces that won't dye black, several that lost the ability to dye at all, saddles that have slid too far back so you sit on the horn, and so forth.
There is but one way these will ever be fixed. You must file a /bug report in the game. Posting on the forums tells the player base about it, but it doesn't send the problem report to the right people at Turbine. Please take a moment to /bug the problems that you find. If you have a few minutes, go through your full cosmetic collection and /bug all of the problems.
The equestrian community thanks you!
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