Check round the base of the shrub, in the soil for black shoelaces. BUT I don't think it is Honey fungus.

now I suspect it is and not all of the honey fungus species [7 according to the RHS] have the black 'bootlaces'.

I lost an old mock orange and old lilac to one of the 7 species. thankfully it was one of the ones that doesn't spread through the garden. The advice the RHS gave me was to dig out the old shrubs and not plant and HF susceptible plants in the area.

cut a twig off and peel the bark back. is there a whiteish layer under the back and does it smell strongly of mushrooms. if it does then that is honey fungus.

welcome to GoY too.