Eximus Stronghold doesn't act like it's supposed to be

[-Serhokyan-]

Hello folks, and eximus stronghold nightwave challenge farmers:
on 26 march 2019 in 3rd sortie mission we had alad v assasination mission with eximus stronghold modifier.
However i have noticed that only about 20 eximus's spawned for the whole session.
after a certain amount of eximus kills no more eximus's spawned.