This is part of the intended game mechanics and current reward system.

Manager cards are distributed from a shared pool that includes all available managers — both those you’ve unlocked and those you may unlock later.
Because of this, even if some of your managers have already reached the maximum upgrade level, their cards can still appear as part of rewards from chests, events, or other sources.

The reward system is designed this way to ensure consistency across all players and progression types. While it may not always feel rewarding when receiving cards for maxed managers, this mechanic ensures access to a broad and fair distribution of cards for all active managers in the pool.