I do not think that the use of the ericaceous compost
could have been responsible for the damage to this Acer
Is it possible that the new compost was allowed to dry out at all after repotting? Reliably moist soil is ideal; as you mentioned it seemed to enjoy sitting in water beforehand. Another reason for the damage could be exposure to strong winds. Did you move it from one location to another when you repotted it? Acer palmatum
thrive on shelter and some protection from full sun which can also burn the leaves especially in high summer.