String roman numerals in 2.22.2

[Matthew Pierce]

New LilyPond user here, hello everyone. Fantastic tool.

Anyone got a method for roman numeral string indications on artificial harmonics in 2.22.2?

This code

  \romanStringNumbers

  \set stringNumberOrientations = #'(down)

  <g c\harmonic>8-1-4\2

gave me an 8th note with a diamondhead [c] above notehead [g] on the staff, and the relevant fingerings [4 above 1] above the staff, but no roman numeral [II] underneath. 

The same code did, however, give me my roman numerals under single notes. What am I missing?

Thanks

Matt