The postscript terminal currently sets an offset used for vertical centering of text only once, in the prolog:
\vshift -<foo> def
This shift is only approximate to begin with and is not adjusted later when the current font changes.
Possible fix (quick-and-dirty): overwrite the vshift definition every time the font is changed using the same approximation used currently.
Better fix: re-write the Lshow/Rshow/Cshow commands to query the current font from inside the PostScript engine rather than using vshift.