White is normally a difficult colour, the pigment is usually titanium dioxide, which is fantastic at stopping the UV light penetrating the resin and thus curing (It acts as a light blocker) Normally the resin requires more power to cure for a given depth.
Its very likely that the white pigment is settling during the life time of the cartridge and thus giving different curing properties over a period of time. If the white settles then the resin will cure through less thickness (higher percentage of pigment at the cure layer) but the area of cure is more diffused (resulting in over cured “soft” rather than crisp edges. The increased white titanium dioxide pigment scatters the UV energy rather than allowing it to penetrate cleanly.
Its a common problem for 2D UV cured printers. (And relatively easy to cure by an experienced print chemist - who can stop the settling)