When changing dpi from 72 > 300, it seems to work in Affinity Photo, sometimes not.
To be certain, I've had to do this resize 72-300 dpi in Pixelmator as a jpg, then import to Affinity Photo as a 300 dpi for export again as a tiff.
A truly odd omission from Affinity Photo.
The eye dropper is a must-have - at present, I'm going back to Pixelmator and Photoshop.
Otherwise I'm starting to like AP, and look forward to more tweaks, especially this one.