Cant reproduce in vanilla minecraft (1.17.1). and as Avoma said , You are using Optifine
Can you provide a clearer detail of your "server" and client? In most cases a debug log helps too. The issue is most probably in your environment.
This is most likely an intended "feature" than a bug. It looks fine in First-Person... Your solution might change the perspective of the way you see the elytra in First-Person View .
This is a duplicate of MC-146524