All bright white paint will yellow slightly with time, even without topcoat. You have probably tried to touch up white woodwork in your home after several years and noticed that the new paint is brighter.
But white and light paints can react if clear coated with a waterbased finish; water-based topcoats are reactive and may draw out substances in the wood such as tannins, dyes or unknown substances in existing finishes causing the topcoat to yellow. This is an industry-wide issue and can happen right away, years later or never.