No reply...
Could I please ask you to tell me whether you can reproduce the
problem without Tk, i.e. what happens if you do
xfd -fn '-adobe-helvetica-medium-r-normal--12-*-75-75-p-*-iso8859-1'
If the problem is not reproducible with xfd, it's a Tk bug, not an X
bug. If it is reproducible with xfd, I will need the output of
xlsfonts -ll -fn '-adobe-helvetica-medium-r-normal--12-*-75-75-p-*-iso8859-1'
Regards,
Juliusz