When you enter sleep mode on your Nintendo switch while the Minecraft application is suspended, you will be logged out of your Microsoft account when you continue playing Minecraft. The game will not allow you to sign back in even if you click the sign in button. The only way to sign back into your Microsoft account is to restart the game.

When this occurrs the game will rompt you to sign into your Nintendo account and not your Microsoft account and then you cannot sign in to your Microsoft account.