Color fonts do not seem to work

[Omikhleia]

\begin{document}
Hello \font[filename=Nabla-Regular.ttf]{World} of \font[filename=Nabla-Regular.ttf]{SILE}.

\font[filename=Nabla-Regular.ttf]

Lorem ipsum.

\end{document}

Nabla font from https://fonts.google.com/specimen/Nabla?categoryFilters=Technology:%2FTechnology%2FColor

Running with -d color-fonts, we do see some actions:

[color-fonts] Switching to color font Shaper
[color-fonts] Switching from color fonts shaper back to previous shaper

But:

• Dubious line breaks (and the relevant code does indeed invoke leaveHmode when switching shapers...)
• No colors

Did I miss something?

[Omikhleia]

N.B. Totally killing the bidi package (= making its code no-op) does not change the result, so this is probably unrelated to #501