The "stitching" of an item is broken, and you can see through the seams. Also, some items occasionally appear transparent.
Linked issues
duplicates 1
Comments 3
This has been reported a lot of times already, if it really bothers you, vanillatweaks has a resource pack that fixes them
Or just read the moderator note on MC-1794 for potential workarounds.
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Duplicate of MC-1794