I took mono green into a raid finally good enough green tiles for all my heroes of which all are fast mana. Elkanen is fast mana hero and has a four star troop with mana increase however everyone filled up with mana except him.
Take a look at your troops assigned to each hero. Some troops give a mana speed bonus and some do not. The percentages can differ depending on level. This could possibly be your missing link? A wild shot in the dark here at least
Mitsuko is a Sorcerer class and so has a chance to drop Mana generation of a hero for 2 turns when she slashes. Maybe she hit Elkanen right before the green tiles cleared?