Going into a singleplayer world that is high in the list, closing and going into Multiplayer, The Server icon on the place the world was will be replaced with The World icon until the servers fully load. This can confuse people.
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Yup, several world/server icon reports already got marked as duplicate of that.

Thank you for your report!
We're tracking this issue in MC-181280, so this ticket is being resolved and linked as a duplicate.
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I believe this ticket is a duplicate of MC-181280.