Are you running Jupyter Notebooks on a Mac or PC?
Also, are you using Anaconda on a Mac? I know from experience there are issues w/Mac’s Catalina and Anaconda (which I still have and haven’t quite figured out yet myself).
Did you try re-running the kernels in the notebook?
Also, I wonder if it’s an issue in how you installed Matplotlib…? Did you use Conda or pip install?