Cores will always print. Soric has less print, but they do always print. It's caused by the larger quantities of resin in the channels of the core shrinking more. When it is cold (I'm assuming you are in Europe), the resin takes longer to cure, so when you demould after 24 hours it won't be quite fully cured and it will print sooner. You can reduce it slightly by leaving the part to cure longer in the mould, but it will eventually print. Post-coating helps reduce the appearance of it.