Thanks, it now seems to work fine in both programs. Strangely GIMP seems to be able to draw a Ü in any case but automatically uses the Glyph from some other font. This becomes clear at least in the current version of monogram since the Glyph ends up about twice as large as it should.
And as a tiny unrelated sidenote for anyone using the Löve framework, use hinting mode ‘mono’ for best results.
Thanks for the fix!