Karoo 2 ”charged” from 50% to 7% overnight

I connected my powered off Karoo 2 to an USB-C wall charger last night and the regular charging display popped up. It had ~50% battery left (but I was leaving for a 150km ride in the morning so needed to top it up). When I disconnected it this morning ready to start the ride, the screen popped up to show a 7% charge. This naturally ruined the day quite badly. What could cause something like this?

The charger is a basic one that has come with some Huawei Android phone. I have seen some Apple chargers simply not start charging a device before, but I’ve never seen one drain a device! Maybe something like endless failed handshake attempts with the device being in a semi-powered on charging state?

Hate having to be paranoid about yet another thing from now on. I already carry an extra cable on my bikepacking trips so that I can mount the Karoo as a storage device on my phone, since I’ve been stranded with a boot-looping head unit on a trip before that was only solved by manually deleting the resumable activity file from it’s storage.