I have a very similar problem. I'm recording a macro in Affinity Photo to reduce an image to a standard size and resolution for use on the web:
Change resolution to 96 dpi.
Reduce image to 650 pixels across, keeping the the horizontal and vertical dimensions in proportion.
Sharpen the image up a little after the last adjustment.
Unfortunately, the macro doesn’t keep the proportions locked as specified (by a small locked padlock icon in the dialogue), resulting in a stretched and unusable image.
I'm using an iMac, iMac (Retina 5K, 27-inch, Late 2015), with macOS High Sierra, version 10.13.5