Charging problem whilst phone is switched on

I’m having trouble charging whilst my phone is switched on. The lock screen shows charging shifting from fast to slow every couple of seconds, and the screen remains illuminated.

If, while charging, I move past the lock screen to the normal screen (showing apps), the dark grey drop-down bar showing shortcuts immediately manifests itself, but the icon images and shades are peculiar and won’t respond. I can’t access any other apps whilst charging, and if I try to switch off, the phone simply restarts itself unasked.

There appears to be no problem charging the phone when it is switched off so I’m not sure it is a cable/charging block problem (although I haven’t got another usb-c cable so I can’t verify that).

I’ve not yet tried a factory reset. The problem only started a week ago, incidentally after the last update.