I downloaded the update as soon as it became available this morning (12.21.19) and immediately found that when the screen is split, although two different documents are active windows, the document only changes on the left side. This happened a few months ago and I believe was resolved by the next update but now it has returned.
Thank you for your help, Jim. Yes, that did it: Navigate--Binder Selection Affects-and make sure current editor is selected. Apparently when I updated to the latest beta, it defaults to something else.
L&L- my apologies for the bogus bug report. Please disregard.