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If you can't turn off the Shuffle mode on a certain playlist make sure you try a clean reinstall in order to get a fresh copy of the latest app version with all necessary updates. You'll find the steps on how to do that here.
It's also a good idea to check if any battery optimization or data cleaning settings are activated on your iPhone - these features can often prevent the app from operating at full capacity. Try disabling them and check again of you can deactivate the Shuffle mode.
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“Music is the wine that fills the cup of silence.” - Robert Fripp