I would think it would be consistent for sedes to follow the calendar changes by Pope Pius 12th since they believe he is a valid Pope then they should also submit to his decisions.
I agree, but many don't. The Holy Week and the 1955 simplifications to the calendar are usually not followed, which is funny because it's a kind of "Sedevacantist R&R". They choose what Pius XII reforms they like and ignore the rest.
As far as I know the CMRI is the only group which is consistent in this area. They follow all the changes that Pius XII did. I don't know if they also ignore the octaves, which Pius XII destroyed in 1955, if I am not mistaken.