I bet it's reading the bootloader status flag, or the factory/non-factory ROM flag and preventing operation is one or both of them are tripped. I sorta understand their desire to prevent their system from being accessed by a non-standard device. I know they're not the only one who takes similar precautions. Some other banking and financial apps also restrict use to official, standard devices.  

 They’re a regulated finance company so this of probably something their compliance department requires. They also disabled Lightning features for NY users based KYC location. 

 I've used cashapp on a graphene phone.

Do you have play services and the store running?