On Wed, 22 Nov 2017 22:44:43 +0000
(Roger Hayter) wrote:

So clearly the side of the engineered wood blocks in contact with the
plaster has expanded very significantly. Is this a known effect of
plaster?

It's a known effect of water.

Would waterproofing the wood first have prevented it?

Probably mostly - I'd have put polythene between the wood and plaster,
and only fixed the wood after the plaster had cured. Or oiled/waxed
the wood (which would be a good idea in a worktop situation anyway).

Will it un-warp in time and is there any way I can encourage it to?

Probably, but you'll not easily accelerate it - applying heat or using
a dehumidifier will affect the front as well as the back, but you want
a differential effect to correct the curvature. Possibly ignore that
on the basis that the wet from the back will eventually migrate to
the dry at the front, and that may happen quicker than just leaving it
to dry naturally.