Dear James, I guess you haven't seen the answer on the TeX Live mailing list, so I add it here.
It seems that this has been the case before, too, and is well known: *** answer from Ulrike Fischer ***** The problem is that \mathbf doesn't use a font with math table and accent commands don't like this. You can find quite a number of questions about this on tex.sx. E.g. http://tex.stackexchange.com/questions/214570/try-to-use-ams-blackboard-bold-font-together-with-texgyrepagella The correct input would be \symbf. Or you must hide the \mathbf in a box: \documentclass{article} \usepackage{unicode-math} \begin{document} $\widetilde{\symbf{a}} \widetilde{\mbox{$\mathbf{a}$}}$ \end{document} (\widetilde{\text{$\mathbf{a}$}} with amsmath work too) ******************** Can you confirm that this *has* worked before, say in the last half year or so? If not, I will probably close this bug report as usage error or upupstream problem. All the best and hope that helps Norbert -- PREINING Norbert http://www.preining.info Accelia Inc. + JAIST + TeX Live + Debian Developer GPG: 0x860CDC13 fp: F7D8 A928 26E3 16A1 9FA0 ACF0 6CAC A448 860C DC13