My 8Plus chimes and vibrates once. And if you plug into a fast charger, it will chime and vibrate twice.
Yes it should vibrate and chime when plugging in. But mine does not do it pretty much every night when I go to plug in before bed. And you know under the usage and standby stats a little message shows up that says it’s been connected to a charger that usually shows up when you plug in and then once you hit 90% the stats go back to 0? Well mine doesn’t do this... the lightning bolt shows up and it charges fine but the message doesn’t show up and the stats do not reset once you hit 90%. They just keep going until/unless I reboot and then I get the normal chime and vibrate behavior and everything resets and is peachy. Never had this behavior until 11.3. And still remains in 11.3.1.
With a couple of iPhone 7 phones that I deal with, since about iOS 10 or so I would only get a chime when plugging in (if the phone isn't set on silent/vibrate) but I would never get a vibrate alert whether or not the phone is on silent/vibrate (and that's with vibrate enabled and even set to be used when phone isn't on silent/vibrate).I have an iPhone 7, and ever since iOS 11 I have this odd issue where I will plug my phone in to the charger and it chimes and vibrates like normal. But then, after about 3 minutes or so, it will vibrate and chime again without touching it as if it was unplugged and plugged in again.
Based on various threads about this kind of thing back in iOS 10 days, if I recall correctly, it seems that that's what others were experiencing as well and it was basically the expected (or at least accepted) behavior since then. Unless that has changed in some way recently, even though it's still the same for my iPhone 7 phones, or it has become different for different phone models for some reason.