Sometimes US providers and unlocked phones don't mix well ̵
According to Android Police, that problem has been solved. The problem seemed to lie with the certification of the IMEI number, part of the network system of tools to authenticate devices and users. Some users may still need to contact Verizon support to enable the non-CDMA devices on the LTE network, but for the most part it will slip into a Verizon SIM card to the OnePlus 8 or 8 Pro now as it should be.  Note that only the Verizon version of the OnePlus 8 is guaranteed to work with Verizon's 5G tires. Verizon does not sell the OnePlus 8 Pro, but the unlocked versions of both phones are compatible with most of Verizon's conventional wireless bands.
Source: Android Police