Please check "Custom Levels" and "Brightness/Contrast" layers on both Photoshop and Affinity to see the difference.
Levels applied in Photoshop only to Red, Green and Blue channels (not to Master) are not present in Affinity Photo.
Brightness/Contrast applied in Photoshop (with the "Legacy" checkbox on) have different values and a completely different look.
custom_levels.psd