Someone explain please....

If it was me I'd remove the dome. One the plant is out of the soil and the baby leaves have unfolded I always remove the dome. Too much humidty can be just as bad as not enough.

Its been removed since the day they sprouted. But the RH is steady at 20% no matter the amount of water I put in to raise the rh.

Try some standing water at the bottom with a heat mat under...should be enough humity then.

No heat mat as of yet. Thats part of the problem.