When finishing basements, our crew has often dealt with varying hieghts in drop ceilings by creating a parting board that is fastened to a 2×4  nailer at the joist and braced on the low ceiling side.  The butts are biscutted and/or splinted (also from the hidden side.)  We use 3/4″ birch plywood, bottom edge banded, and pre-painted for this”soffit” board, . Wall angle is then stapled or screwed on each side.  It’s worked great for over 5 years.  We recently tried MDF on 4 jobs.  Our only callback was on an MDF soffit that we did not pre-paint. The owner handled painting before we hung the ceilings.  2 months later the wall angle had twisted or coiled along the soffit on both sides. The ceilings were not bearing on the wall angle and the soffit board remained straight.  Can the MDF shrink?