The bug:
Attempting to join after uploading an updated world to my realm it fails to join until "resync" or in other words restarting MC.
What was expected
Upload a new copy to Realms and be able to access it.
What happens:
Tells me "failed to connect to server" or similar sayings..
To recreate:
Open a realm and upload a new copy of the same world, then attempt to connect to it.
Solution:
"Quit" Minecraft and relaunch it from the home screen.