Measuring tool not calibrating correctly on Mac OS Big Sur

I've been using the measuring tool for many years with no trouble. But after installing Acrobat Version 2021.001.20145 on a new Macbook Pro with Apple M1 chipset running OS Big Sur (version 11.2.3) when I set the scale ratio and then start measuring a pdf, the final measurment is never callibrated with any units (let alone the correct units) and the numbers are incorrect. I attach two images. The first shows the correct calibration immediately after I set the scale ratio, the second shows what I see when I finish measuring an area, just numbers that bear no relation to what I know the area to be (that I've measured elsewhere). 

Many thanks.