It didn’t have enough medium in that solo cup to begin with ( probably depleted some of it nutes ).
up pot as stated , maybe dust a little myco on roots before transplant and use straight TAP - no measuring ph. Its a plant just like a houseplant and your outside lawn , it doesn’t need “ adjusted “ water.
A larger container of medium ( ocean forest for example ) is prebuffered and will compensate for the plant. Plus will feed that plant alone without adding anything.