Coffee and herbal teas may not be as beneficial during fasting, because as you said, they may be more "substantial".
I was watching a compilation clip about intermittent fasting [0] recently. According to Dr. Rhonda Patrick, coffee and other drinks than just plain water (and maybe vitamins and supplements as well?) break the fast by starting up some metabolic enzymes [1][2].
I guess there goes my round the clock tea gulping. :|