This is intended behaviour. PWO follows the standard convention of Pokémon games, whereby Pokémon cannot exceed Level 100. As a result, once a Pokémon reaches this level, all experience it gains from that point forward is ignored. (However, a Lv100 Pokémon can still continue to gain Effort Values, if they have not reached the maximum 510 EV Points yet — meaning their stats can still increase even once this level is hit.)