"The fatwa volumes display almost a paranoia about the tilak. Mufti Kifayatullah’s general position is that it is the mark of kufr, but he relents to the extent that he makes the outcome depend on intention and circumstance. Hindus and Muslims are in a procession together; supporters of the cause put chandan tilaks on the participants, including some Muslims: are the Muslims guilty of kufr?, he is asked. Is their nikah with their wives terminated? ... To put chandan and marks on the forehead is the special national and religious characteristic of the Hindus, he says. It was obligatory for those Muslims, he rules, to abstain from this. But, he adds with his usual moderation in regard to specific transgressors, because we do not know whether the chandan was put on those Muslims with their willing approval, one cannot decree that those on whom chandan was put are apostates or kafirs or that their nikah has become void. However, he concludes, they should do taubah and abstain from such actions in the future."
January 1, 1970
https://en.wikiquote.org/wiki/Tilaka