The cape is not hidden when wearing an elytra with armor, so both the elytra and cape is shown.

Thanks

OptiFine capes

Do you mean the mod OptiFine or the fabric mod called of-capes?

Are you talking about vanilla capes or capes added by a specific mod?

the mod optifine

Due to Optifine being a forge mod and requiring optifabric for fabric support and Optifine being closed source and thus more difficult for me to override features (e.g. temporarily disabling the cape). As a result of this I am not going fix this issue.

I'm pretty sure this mod does work with of-capes and I'll be happy to fix any issue relating to that or other fabric mods.