I see two problems here:

1. The strike-through in our case is placed at half the height of an
   upper-case glyph.  LibreOffice seems to have a smarter positioning.

2. The strike-through line is hard coded to 1 pixel in our case, whereas
   LibreOffice scales it up as the font size increases.

I'm have no idea how to fix it; the font code is pretty opaque to me.

I mucked around a bit and came up with the below diff.  It is probably
not the correct fix, but does make it slightly better for me.  Someone
who understands this better would need to investigate this properly.

FWIW, here is what it looks like with the below patch: