I'd guess that the MG was the problem.
It's too hot for seedlings.
The replanting was a good move but quite stressful.

We usually love our first grow to death.
Cannabis is a weed, it thrives on neglect.
Very hard to diagnose without pictures but they sound over-watered.

Next batch, fill a container with dry soil and weigh it with your hand.
Then water to run-off and heft it again.
Remember the feel of it and use that as your watering guide.
(My memory is dodgy so I keep a pot of dry soil on-hand as a reminder.)

Not too wet, not too dry,
roots gotta breath
and that is why,
we mix da soil until it's right
with neutral PH and vermiculite.

Sorry, best I can do with the info supplied.
Suggest that you rat about in the everyt'ing link in my sig, while you wait for responses.

Aloha,
Weezard