The PCOMP command is for displaying the loadings, scores and biplots. MAKEPCOMP is for saving the scores to the workfile. The syntax that you used in PCOMP has a potential conflict with the display of loadings when producing biplots. It works right now because the command maps to the same code as MAKEPCOMP, but that might not be the case in the future.
As the documentation states MAKEPCOMP is the method of producing the scores.
To compute principal components scores and save them in series in the workfile, see Group::makepcomp.
I do see that there is an example which shows your usage but that's a holdover that probably should have been changed when we added additional features to principal components. Either than or we should officially support your usage. I'll put in on the list of things for us to consider.