The items that can have all the millions of colors have some invibible bits that should be 3D but aren't. I've noticed that it occurs especifically with the parts that keep their colors when everything else changes theirs.
MCPE-42415
Items that can change colors have invisible bits
Resolved
Fixed
2
(Unassigned)
182991
Confirmed
Android
Android 7.0
1.9.0, 1.11.0.1 Beta, 1.11.0.3 Beta
1.11.0
Retrieved
Fixed in 1.11.0.7 beta - Thanks!
That was a really old bug that affected leather horse armor since 0.15, and then firework charges in 1.2 and leather armor in 1.10 (caused by the new textures). I'm really happy about this tiny fix!