When NLA strips weren't time-aligned with the underlying action, then fcurve modifiers would not be drawn anchored to the strip. Fmodifiers were evaluating properly, they just weren't drawn with the proper offset and scale.

To fix it in this specific case, I've chosen to undo the keyframe remapping then remap the draw-evaluation-time from scene time to fcurve time. Afterward, I redo the keyframe remapping so the controls are properly drawn.

The Envelope fmodifier has special drawing code which was fixed too. In this case, no mapping at all was happening. The solution was similar, to remap the envelope control points from fcurve time to scene time.

Quick test file.