OWC is reporting on what is obviously a bug with M1 Macs, because you shouldn’t have to connect a second display to get full speed write on the thunderbolt ports, but at least for now it seems you do.
Man Big Sur has been having some serious software issues when it comes to drives (losing support for SoftRaid for a full versions was huge!).
EDIT:
Since writing this OWC has figured out more ways to get the speed boost, check it out.
