If the forsythia begins to wilt during bloom and subsequent foliage immediately dries, it is likely a clear infestation of pathogens.
The cause of the so-called forsythia wilt is usually the fungal pathogens of Monilia disease.
This shoot tip wilt occurs preferably in years with showery weather during flowering and shortly thereafter. Then the fungal spores infect the still soft shoot tips and the fresh leaves wilt.
The damage can best be limited by pruning.
Since Forsythia blooms young anyway, the affected tips can simply be removed with the accruing plant pruning at the end of May/beginning of June. This involves cutting off the wilting and dead shoots and disposing of them in the residual waste. Do not throw the plant parts on the compost. Otherwise the germs will infect other plants and the fungus will spread in the garden.