Cilic was challenging a lot and late - imo Zverev was right to complain about that. One should challenge immediately and not check out the mark first (too much opportunities for secret coaching on the challenge). But Cilic paid a price for it... he ended up with no challenges (even the extra one in the TB was gone rapidly), and when he wanted to challenge his second serve (called out) , resulting in a 5-1 lead for Zverev, he couldn't anymore.
And review showed that the ball was indeed touching the line. So instead of 4-2, he trailed 5-1... and two points later the match was over (7-6 7-6 Zverev).