Hello, I have a Samsung Galaxy S8 (AT&T) running Android 8.0 and Samsung Experience 9.0. I noticed this happening a few updates ago and it's been driving me crazy.
Even if I turn vibrate off in the app info settings, the notifications from the app still make my phone vibrate. I basically have to turn notifications on completely to get it to NOT vibrate, or turn the phone to silent (or turn do not disturb mode on but I would rather find a way to fix this than run that 24/7). I have noticed this happening most with FB Messenger and Telegram since i get messages from them the most, but it happens in all other apps too.