The sub floor should be dry then. Weird that. You haven’t had a leak from above I take it.
Any old water pipes close by or under the screed ? Other than the UFH.
Order should of been
Screed goes down
Leave 30 days
Switch on UFH low temp to force dry the screed for 30 days
Moisture test
If dry smoothing compound , 2 days later lay LVT
If still got damp in it switch heating back on