Whenever the switch enters sleep mode or internet connection is lost and a reconnect is tried, the sign in service fails or is stuck in a loop requiring a restart of the game to fix. Thankfully after the Switch version 10.0.0 the game launches much faster, but this is still very annoying and a fix would be greatly appreciated! This has happened across all of the wi-fi connections I have been connected to. The following image shows the generic sign in loading animation (It somewhat often goes invisible as well)

Steps to recreate:
-exit the game to the home menu (but do not close the game)
-leave the console in sleep mode for a minute
-Reenter the game and go to the home menu hitting sign in to nintendo account.
The sign in will fail and the text "Signing into Nintendo Account..." will sometimes appear or not at all.
The Nintendo sign-in process should be stuck from there and not resolve itself.

(Note that restarting the game fixes this issue temporarily)