Is undercoat / sealer necessary for new plaster?
The others are all right. I think the problem lies elsewhere. Is the
different texture between the old and the new plaster giving the
appearance of a different shade? in which case you will need to go
over it all with lining paper.
Or maybe the existing paint on the existing plaster is leaching
through the new and discolouring it, this would happen if it were
yellowed for example with nicotene.
Just some thoughts.