Hi Rob,
Please try disable the hardware graphic acceleration in the Excel > File > Options > Advanced > Display > Tick on the following option.
See if the issue persist.
Meanwhile, please help confirm the following information with us:
1.Please provide a screenshot on your Excel Product information(Including the version number), you will find it at File > Account.
2.If you maximize the Power Query window, will it display properly?
Regards,
Alex Chen