Rockwool strikes again! Probably not a bad cube, those things are just tough to get the hang of. Once you do they are apparently great but the finess has always eluded me.
Great growing medium with some practice, but I think the margin for error is so slim I always steer people away from them, especially when rapid rooters, root riots, regualr starting soil and even jiffy pellets seam to give more people more success. The forums are filled with people killing seedings or not being able to germinate seeds in them. Not saying they are bad, just hard to master.
I've used rockwool cubes with success and never had an issue NOT pre-treating them or soaking them. I just ph my water I give them to 5.5-6.0