Not sure just how long the buffering capacity of seedling mixes lasts. But I'm pretty sure they are buffered to a lower ph to begin with, and the ph crashes when those buffers run-out. My guess is that the manufacturer adds just enough limestone to last a seedling till it's first transplant into a longer-term mix. (perhaps a couple of weeks...?)

Likely best to save what you have for germinating seeds, and get some 'regular' potting soil for the rest of the plant cycle. Better particle size for maturing plants, too.

I prefer perlite, but both are usually available at most nurseries.

Personal Opinion Section:
I have no problems with using organic products if they are a cheaper product. But to me, going organic is secondary to what works, and what I can reasonably afford. You don't need 10 different bottles of organic crap to grow quality meds.