I have a highly regarded, but old 50/1.4. I also have a cheaper 50-300/3.5-4.

Big surprise - the 50-300 zoom looks MUCH better than the 50/1.4 at 50 mm. I'm talking about the colors, they are just better and more natural (less aberrations probably). Of course, if you have enough light since it's slower.

I investigated this, and it turns out modern lenses have much better coatings and other optimizations.

Conclusion: lens age matters too, if it's an old design investigate.