Step 1: Sign in to the Realms with "Minecraft on Switch".
Step 2: The add-on will start downloading.
Step 3: After you have signed in to the Realms, quit Minecraft.
Step 4: Sign in to the Realms again.
Step 5: The add-on has not been downloaded. Then the download starts again.

Infinite loop. The add-on will not be saved forever.

And the add-on doesn't work properly only for "minecraft on switch". That's the biggest problem! It's gone break since 1.16!

The windows 10 version and the iphone version are fine. Please fix the Switch version as soon as possible.

P.S.
If you have the same problem, please comment. My friend has the same problem.