No, grout is not an adhesive to make the tiles remain in place. It is one part of the tile setting system and is necessary for proper installation. However, it's purpose is not to make the tile adhere to the substrate. Your problem is with the thinset. It is either defective (not likely) or was not mixed properly, or was applied too long after mixing. You probably should scrape off all of the ceiling tile before what you have sets up, clean up the tile and the substrate. Mix up a fresh batch of the thinset (you didn't use that pre-mixed stuff from the big box store did you?) carefully following the directions, then reapply the tile.
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