[Please refer to the attached image.]
4. The space on the right side of the plus seems too wide.
5. The first note seems too close to the time signature.
Lacking any response (-confirmed/existing/solved in newer version/not- bug),
I extended the "hack" for compound time:
#(define (compound-time grob one two three four)
(interpret-markup
(ly:grob-layout grob)
'(((baseline-skip . 2)
(word-space . 2)
(font-family . number)))
(markup
#:line
(#:column (one two)
#:lower 1 "+"
#:hspace -2.4 #:column (three four)
" "))))
which makes it even uglier (see the negative "hspace" and the added
blank space), and still it will only work for default font size.