In ClipStudio Paint, in ArtRage, in SketchBook Pro, In Photoshop you find the function of filling all layers below the ink layer. If you are in the line art illustration habit, flood filling flat colours on by one on separate layers, afterwords adding tints and shades and some texture, Flood Fill on underlying layers is a must. The line art remains untouched, the colour layers can be masked and evryting gets so easy and fast. I miss this workflow and feature in Affinity Photo and Designer.
I really like Affinity Photo as a painting / illustrating program. My suggestion is: add this feature. It will be one more reason to switch over from Photoshop (or any other program)...