MacBook Pro 16" not all ports working properly

I got the new 16 inch MacBook Pro and so far its great, I just noticed that my external monitor won't connect when I'm using one of the usb-c ports on the left, but it works perfectly fine with the other two ports on the righthand side. So now I'm wondering if it's a software bug or if my ports are broken. I thought about it being a preference problem but I didn't find anything about it and I think it wouldn't make sense to even have such an option, would it?

So I hope any of you guys have a solution for this, thanks in advance :-)

Btw: I'm using a Dell U2518D with a Displayport to USB-C cable.