This thread is a few months old, but I recently realized I'm having this problem too. I tried all the solutions here and in other places as well. I finally figured out the solution for my phone at least. Under "Vibration Intensity" in Settings, the Notifications level had been turned all the way off. I don't know how that happened because I definitely didn't change it myself, and no one else I know would've done it even as a prank. But that has fixed it for me! I already turned it back up when I took these screenshots but I wanted to illustrate where in the settings this is.
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