The example can be further reduced (and plainified) by removing the preview package -- a simple \vbox is enough:
\input luaotfload.sty
\font\x="texgyrebonum-bold.otf:color=FF000077" at 24pt\x
AAA \newbox\y\setbox\y\vbox{AAA}\unvbox\y
\bye
The effect is that the setting of the pdfpageresources (which must contain the transparency spec) does not propagate out of the vbox. (What I do not quite understand, though, is why there is no transparency even though the transparent font is also used outside of the vbox.)
The pdfpageresoures are set in luaotfload-colors.lua, in the function color_handler, near the end of the file. Changing the line
Debugged by Robert on tex.sx: