Interesting. This is happening to my potting soil as well, but mine was promix straight from the bag. Everytime i water, there is a fair amount of foam that bubbles up. I didnt make much of it when i first started using it, but did note that there was a fairly large amount of nitrogen in it because of the really dark green leaves that appeared, shortly after i started having problems tho. I made a slurry and tested the soil, its in the 2.0 ec range. Thats a slurry too! I think it may have something to do with some nutrient or chemical that is in the soil.
Idk. Felt like giving out my story in case anyone else is having the same problems. Cheers.