My Nokia 7 plus also not perform fast charging. It takes more than 4 Hours to get full charged in idle position (no one touch phone). Fast / rapid charging not appear on screen. I am using original charger and cable. While in power off position it get full charge in around 2 hours. It is definitely a software issue and must be fixed in next software update.
Nokia 7 Plus only gives fast charging in power off position. While in normal position, even screen is off, it takes more than 04 hours to get full charged. That in useless. Other brands like Samsung, LG provide fast charging in normal position. The problem must be solved on priority basis through next update, otherwise Nokia should remove fast charging from Nokia 7 plus specs.
I have an extremely similar problem, when I turn on the charger, on the lock screen it sometimes flashes between 'charging' and 'charging rapidly' and other times it just says 'charging'. Sometimes it feels like hours before it's fully charged and others times it is very quick. I'm am not sure if this is software since when I plug the cable in its is very slightly wobbly. This is very annoying since I got the phone recently. Thank you for bringing this up