Image gets resized when switching size units

If I drag an image into a score, then tick/untick ‘Size in stave space units’, then image resizes. Instead of converting the size from ‘[x] sp’ to ‘[y] mm’, it converts the size from ‘[x] sp’ to ‘[x] mm’. The numbers it show suggest that no resize has occurred, until they are refreshed by clicking somewhere else on the score, then back on the image.