This is the time where the Requiem of The Echoing Depths Archon quest was happening, when the Abyss Herald was activing the device it affected all Khaenri'ahns, that means it also affected Kaeya

Kaeya was at Angel's Share alone while Diluc was mixing some drinks and it was just the two of them since it was about to close

When suddenly Kaeya's head got a massive pain and Diluc was facing his back so he didn't notice Kaeya