Should a Sephardic davening in an Ashkenaz minyan for shachrit on Tisha B’Av wear tefillin for shachrit or later for Mincha?
Somebody whose custom it is to put on tefillin for Shacharit on Tisha Be’Av should nonetheless not do so in a minyan where people do not do so, for fear of “lo titgodedu”. Rather, he should put on tefillin at home before davening, and then come to shul without tefillin.
Note that many Sephardi communities also refrain from putting on tefillin on the morning of Tisha Be’Av, as the Shulchan Aruch (554:1) rules.
See Mateh Yehudah (555), who mentions the issue of “lo titgodedu” in this context.