I've had that happen when I used plastic pots and trays because of the access water that would stay towards the bottom and in the tray.
I didn't know it was good, but I guess now that I know about all the teas and fungi teas and such, it probably was a good thing. I thought it was bad previously.
Hmmmm.... That's got me thinking about my old soil mixes I made before I knew about all this stuff. I don't even mix my soil anymore, its all ffof or recycled ffof