Armors pieces icons and items icons such as shields receive a fake overlay or fake render when exchanged for an item of the same type in a given slot.
Examples:
-Shields: shield with banner in offhand - swapping it for a new shield without a banner causes the shield to acquire an appearance on its icon that makes it difficult to differentiate between the two shields.
-Armors: Diamond chestplate with armor trim - exchanged for a new diamond chestplate without armor trim. This, as with the shields, Causes chestplate icons to have incorrectly rendered pixel art. Since there is no love trim applied to the item
Note: Please don't ignore this bug. It's very tedious to make custom shields and not know which one has a banner and which one doesn't. Please, Mojang, fix this.
You already reported the shield issue at MCPE-187303. Please don't create duplicates of your reports. The armor trim issue is being tracked at MCPE-187679.