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First have your tried blocking the text number? If it's coming from the same number you might be able to block it.
Everything else is a question well mostly. Everything you describe sounds like a bad app. Try booting your phone in safe mode, it should be just like a restart so it should result in the same SMS you say you're getting whenever the phone restarts. You'll need to let us know if it happens in safe mode.
Speaking of which, you really shouldn't let low battery turn your device off. It could corrupt your OS or some of your apps, is really bad for your device.
Do you currently have an Oncircle app or account? Looks like something my that came on the S6 devices and it looks like sanding shut it down but off the app is still on three device you may need getting three notice, just because. Some it's from Samsung I'd have to say it's not malicious just on the face of things. Try disabling the app. Read mate about it here...
https://www.sammobile.com/2016/11/02/samsung-shutting-down-oncircle-next-month/amp/
Was this device new, used or a hand me down from a family member? They could explain sooner of the odd things too.
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