Description: Experiencing a dsync issue while including the latest fabric-api mod.
Effect: Items are rendered on the client incorrectly, (i.e Ghasts swapped for foxes, Elytras are rendered as shields, etc...)
Workaround: The workaround described by @modmuss50 is to delete the following files (please take a backup first)
/.fabric/ directory/world/data/fabricRegistry.datHmm, so only deleting .fabric doesn't work? That part is loader-centric. The registry dat deletion is a fabric registry issue then.
Is this happening when connecting to a remote server, or does it happen in single player?
I think the way to reproude this is:
create a 1.14.4 server with only fabric-api
update that server and world to 1.15
While connecting with a vanilla client.
Maybe related with #135
I also had a similar issue but with blocks while updating my fabric server from 1.14.4 to 1.15 (mods: carpet and carpet-extra). Using a vanilla client, Blocks such as Smokers and blast furnaces did not render or rendered as other blocks such as lower spruce doors (and redstone dust once).

Duplicate of #122
Most helpful comment
I think the way to reproude this is:
create a 1.14.4 server with only fabric-api
update that server and world to 1.15
While connecting with a vanilla client.