Another track related backstory found here is for "Luigi Circuit", the first track of the game.
After finishing the events of the original Luigi's Mansion, part of the gold/money he found there went to the construction of another circuit under his name. 2/2

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This decision becomes more confusing when you notice that the release of Splatoon 2 ost came with all the Octo Expansion tracks, which just like Side Order, is a DLC from that game.
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However, yesterday, it was confirmed that these extra tracks weren't planned to be released and were accidentally left in the search feature by accident.
They were removed with a patch and are not available to listen to anymore.