I have had this issue the other day on my FP5, and my friend who has a FP4 has had this issue at least once or twice. I believe I have had this problem previously as well.
I tried finding any recent info about it but couldn’t, so I figured I would post about it here.
I am running my phone in vibration mode 100% of the time, with few exceptions where I switch it to silent mode. At times, the vibration on incoming calls gets disabled, but is shown as enabled in the settings.
My friend with a FP4 has also had issues with vibration not happening even though the phone is in vibrate mode.
For me, this time, the solution was to disable and enable the “vibration for phone calls”-setting.
Is this a known bug? Since me and my friend experience it a few times already on two devices, I expected to find more recent info about this somewhere..