Rehgar’s new 20 talent, Pit Fighter, should give a bonus starting at 1 and adding +1 per friendly hero near, up to 5. But when a friendly hero dies nearby, the +1 bonus remains permanently. When that hero respawns, he can provide the bonus again, granting +3 with just one hero near.
If he dies again, you can repeat the effect, eventually gaining 5 Pit Fighter while completely alone. (Murky synergy)
The bug will end if you gather with your whole team and reach 5 Pit Fighter legitimately. Then the extras will be removed.
PS. Tassadar’s 20 talent Khala’s Gift also gives a bonus per nearby friendly hero, but it is properly removed when they die.