Highlighting an image with a graph seems to change the styles of the graph vertices:
img = CloudGet[CloudObject[
"https://www.wolframcloud.com/objects/f28eb939-485b-4899-8ff7-e6c7d64cef60"]]
mg = MorphologicalGraph[
DeleteSmallComponents[Binarize[RidgeFilter[img, 3.5]], 20.7],
VertexSize -> 7, VertexStyle -> Yellow,
EdgeStyle -> Directive[Red, Thickness[0.01]], ImageSize -> Large];
Row[{mg, HighlightImage[img, {Show@mg}, ImageSize -> Large]}]
Is there any way to prevent that?