Not underwatering

Yes, you better check your PH..
It looks like your new growth is doing well... but the plant is necrotic in the middle..
correct?

There are probably a few nutrients the plant is not getting because the PH is whack
My guess it is an MG deficiancy... but adding more MG will porbably not help your plant... it is the whacked PH that wont allow your baby to absorb proper nutrients

love

If possible show pic of entire plant..
: )