Your old MB was likely charging at default USB-C port which "should" have a minimum of 7.5W charging, i.e 3x of what you are getting. My guess is that OnePlus is looking for USB-PD 1.0 but your newer Motherboard only support USB-PD 2.0 and 3.0. So it somehow dropped back to the minimum USB2.0 charging standard.

Again you should look at BIOS settings to see if there are any relevant options.

Regular charging is only 2.5W so it is technically possible for the phone to use as much as that while you keep the screen on and run a heavy app. Leading to what you describe, the battery never reaching 100%. It will of course also require hours to charge at that speed. Check your battery graph. If it stabilizes there may be a background app using a ton of power.

But yeah fast charging a 5T is never going to work without its own charger. Only the latest OP models can do that and even then not as fast as they can with their own charger.

If you want this get another brand next time. Although most brands are now moving away from standard USB-PD for everything and require USB-PD with PPS which allows fast and fine voltage adjustments. USB-PD has only a few set voltages and needs an interruption and slow negotiation for every change in voltage. PPS can do fine adjustment on the fly.

The reason that fine adjustments are needed for ultra-high charging speeds is to remove the bulky and heat-generating high-power DC-DC converter from the phone itself and move it to the charger where heat and space are not so much of an issue. It's basically what oneplus has done for years but now in a standard way (and not compatible with OP's implementation)

No laptop charger or port I know of supports this USB-PD-With-PPS right now. It'll still charge medium fast though. 18W or so (which is not actually a lot in most modern phones that have speeds of up to 100W sometimes)