Working Joycons not charging when plugged into Switch

I have a Switch that seems to work perfectly, but it doesn't charge the joycons.

The Joycons will be detected by the Switch, and both will work normally during gameplay. They just don't get charged by the Switch. The Joycons will charge normally if plugged into another Switch, so the issue appears to be with the Switch itself.

The Switch will charge itself without issue if plugged into the docking station or mains. It just doesn't charge the joycons. I've tested with mutiple joycons on both sides on the Switch and no Joycons charge on this particular Switch.

I've cleaned the Joycon connectors and their respective charging ports in the Switch with Isopropyl, but this has had no effect. I've searched online and found one repair suggesting it's caused by a diode connected to the fan, but looking into this i have a later model of the switch that doesn't have the diode.

Can anyone offer any suggestions? Switch does work fine other than this so when i sell it on i could just include an external Joycon charger or something similar, but i'd rather get the issue fixed.

Thanks in advance.