As the one who set up the formation, Riya was very clear that the formation would not last long to block the cracks.
And when she was in the crack, Riya didn't really eliminate all magic beasts that exited there. She simply hunt those that she met on the way towards the pillar. But afterwards, she came out and didn't explore the entire crack completely.
Based on the crude map that she had made, there were many intersections that she didn't explore.
From these alone, Riya could guess that there might be more magic beasts.
But she had no intention to go there.
Especially when she was heading towards the second pillar through the first pillar's clues and map route. Riya completely ignored the other paths and heads straight to the second pillar.
Afterwards, she left the crack.
So there were definitely still many magic beasts left there.
