Hi, I am a Newbie to Affinity Photo, but have had years of amateur experience with Photoshop CS3.
I am experiencing the same or a similar problem with respect to the Levels histogram.
If I open a JPEG photo and create a Levels adjustment the all is well and the Levels histogram is presented and I can adjust the sliders to affect the whole image.
If I close that image and start over, i.e. I open the same JPEG photo but this time I make a selection of just that part of the image I want to adjust, when I invoke the Level adjustment the histogram is now blank.
I have waited 5 m