Abyss closing with no rewards

I'm main farming abyss, doing 2-4 per map, and this bug has been happening for something around 10% of the abysses (which is a lot if investing in scarabs).

What happens is, when the abyss stops to spawn monsters, it spawns a very little amount of mobs and then it proceeds to close the hole and end the abyss without any rewards at all.

I couldn't find a pattern whether it happens more on the 1st, 2nd, 3rd or 4th opening, but I did notice that the frequency raised when using gilded scarab, so I assume it has something to do with a spire spawning bug.

Sometimes, right before spawning a spire, I also notice that the mob spawning frequency is lowered by a lot. Maybe this is causing the holes to close with no rewards.