You just can't ask somebody who's never played guard to play guard. He played right tackle his entire college career. It takes some time to adjust your footwork, stance, hand position, etc...
I'm with you. His two best years came at right tackle (yes I'm using pff as a source). He was below average in 2019 as a guard for the Panthers. The only way I see Spencer Brown getting playing time is an injury to Dawkins or Williams.
It’s pretty odd so that's all that I could think of right away. I guess you could say McBeane want their practice squad players to “earn” that designation.
I guess it’s also possible McBeane are being nice guys and allowing the remaining ps players to have the opportunity to be signed to another 53 man roster as being protected is punitive in some regard. It's little things like that that builds trust throughout the organization as a whole. Maybe I'm reaching.