To me it seems weird, that all your heroes were just missing ‘one tile’.
Just to be sure: could you please check the opponents’ team, whether there is any hero with a mana reducing effect (e.g. Li Xiu, Neith, Mitsuko)? As @JonahTheBard already suggested, it seems very likely that your heroes were completely charged and after that a small portion of mana was stolen from them.
I just tried to reproduce this effect in raids and farming. Everything worked absolutely unsuspicious for a team composed of very fast (Jackal, Malosi), fast (Poseidon), average (Onatel) and slow (Vivica) heroes. I checked for mana and crit troops as well - no obvious bug in that tests.
However, there was a bug with four small mana potions not loading very fast heroes a few weeks ago. It seems very unlikely that there is any connection to that issue, but I wanted to mention that one. Maybe someone sees any relation to that bug?
Did you try to reproduce that effect in another surrounding (i.e. raid, farming, titan)? It would be very helpful to see a video or at least screenshots to help you solve this issue. Same question @Lordangeleye ?