So, Finally is it a bug in MKIV? It seems you fix some kind of bug in your messages. Is it true?
The output with Luametatex is correct but Luatex creates a raised bar (and different linebreaking).
\showglyphs
\startTEXpage[offset=1ex] \ruledhbox{$\overline {OA} = \overbar {OA} = 4.2$} \par \ruledhbox{$\underline {OA} = \underbar {OA} = 4.2$} \par \ruledhbox{$\widetilde {OA} = \widebar {OA} = 4.2$} \par \ruledhbox{$\overbracket {OA} = \underbracket {OA} = 4.2$} \stopTEXpage
I'll add a 'force fix in spite of extensible being present' in mkiv. In lmtx we always fix also because we drop in nicer shapes (watch the round tips in lm and fancy tips in antykwa) and concluded that fonts will remain as they are.
Hans
As you suggested, I will give a try to LMTX but be aware most user still use MKIV until most distros upgrade the texlive version of context to LMTX. Regards, Xavier