over in the "Verizon Certifies Unlocked OnePlus 8 and 8 Pro" discussion:
The Verizon OnePlus 8 is the only model in the series that supports mmWave 5G. This is what allows the device to support Verizon’s Ultrawide Band network. OnePlus had to place the mmWave antennas on the sides of the Verizon model, which meant moving the volume buttons slightly. You probably wouldn’t notice this if the different models were placed side by side, but it’s enough to make cases not fit correctly.
It’s unfortunate that users will have to be extra careful when buying a case for the OnePlus 8. If you own the T-Mobile/unlocked or Verizon models, make sure you take note of this when searching for a case. 5G has apparently brought back the days of carriers receiving different hardware for the same products.