Affinity Photo - 1.7.1 on iMac (Retina 5K, 27-inch, 2017 - Apple Identifier 18,3) with 24GB Memory running Mojave 10.14.5
Printing N-Up. Although I can change all parameters using the spinner, trying to type in new values is difficult.
1:Single click in any Box and the value disappears! Typing cannot be seen and often doesn't work to change the value. You can only see what has happened when you press Enter. In the Across/Down and Repeat/Skip fields, any additional typing is appended to the number already there - unless you press Delete to remove the digits - which you can't now see!
In the Left/Top and HGap/VGap Fields it's even more complicated - because of the presence of the unit.
2: The only reliable way I can find is to DOUBLE-click on the NUMBER. This highlights the number, but as soon as you type a new value it disappears, and the new input is only made visible when you press Return.
3: I would expect the following behaviour to be more logical. Single click in the box highlights the Number. This can then be changed by typing, but with the input visible at all times and becoming fixed when you click out of the field - no Enter key being required.
Similar Problem occurs in Publisher!