the problem is not limited to just legacy documents which have been loaded into a newer version of Inkscape. It also occurs if you modify the Scale in the Document Properties dialog. Attached is a new drawing created in 0.92.1 in which the scale was modified to be 1.5, and then the Perspective was tested. The modified perspective drawing is well off the page, one must zoom out to see it.
confirmed on Windows 10, Inkscape 0.92.1 r15371
the problem is not limited to just legacy documents which have been loaded into a newer version of Inkscape. It also occurs if you modify the Scale in the Document Properties dialog. Attached is a new drawing created in 0.92.1 in which the scale was modified to be 1.5, and then the Perspective was tested. The modified perspective drawing is well off the page, one must zoom out to see it.