I have used BonusPrint, BonusPhoto, DoublePrint, TriplePrint (all the SAME company, Grunwick Laboratories of Borehamwood, Herts) for many years. Recently I suspected the prints were being done digitally but on standard RA4 colour paper process. I have NOT seen the effect you describe, and all of the films I send are well out of date!
-------------------- Got COMPUTERISED at last and now Digitised but FILM still RULES!