I found a fix for this error.
It appears that the different aspect ratio of the second display confused InDesign.
I closed all docs, then switched the arrangement of my displays to put y desired aspect ratio on the left.
When I reopened the document I was working on, the aspect ratio was correct and the frame filled my working display.
This must have reset some parameter or preference.
When I put the displays back to their original right-left order, the frame remained correct.
I found a fix for this error.
It appears that the different aspect ratio of the second display confused InDesign.
I closed all docs, then switched the arrangement of my displays to put y desired aspect ratio on the left.
When I reopened the document I was working on, the aspect ratio was correct and the frame filled my working display.
This must have reset some parameter or preference.
When I put the displays back to their original right-left order, the frame remained correct.