I'm having the same issue on a Google Pixel 2 XL running Android 10 and Pandora Plus with app version 1910.3. Mine does pretty much the same thing where it starts playing after last being used anywhere from 15 minutes to an hour or so. I do use Android Auto but I haven't narrowed down if that is related. I've had to remember to turn my phone volume off so it doesn't start blasting music while I'm at work.
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