Cito,
I am not familiar with the substrate you mentioned. I have used vermiculite before with great results. It probably is the same thing.
The bugs may be attracted to the varying levels of humidity in the container that your eggs are in. How much water do you have mixed with the substrate? Too much may lead to bugs arriving, like you described.
Back when I bred, I had a few of those buzzy little fly like bugs a few times, but I just got rid of them by opening the lid and letting them fly out. This only happened about 2 times.
I have also seen those bugs when I would be cleaning cages and found a mess that my snake left me that I didn't see and the moisture from their crap attracted some of those little flying bugs.
A buddy of mine had maggots in his vermiculite that was filled with his snake eggs. Turned out his vermiculite was WAY too wet, so that and the dark closet they were in made a recipe for maggot substrate. LOL!
I kept my vermiculite perhaps drier than others. The amount of water I put in was like this. I mixed it with the vermiculite till I could take a handful and squeeze it into a clump that stuck together. It was a little moist, but not too bad.
Eggs always sink in a little before they hatch. Don't worry bro. I thought the same things you did.
Take care! Hope this helps!
Billy
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Genesis 1:1