When trying to equip the banner on an airship :

• It appears for the player who places the banner
• It doesn't appear for the other players
• If the player who equipped leaves and goes back in the airship, the banner isn't here anymore and is back in the player's inventory

Minecraft 1.20.1
Forge 47.0.19
Immersive Aircrafts 0.5.1

Most likely related and fixed with #80, if not please respond here