Originally Posted by FBinNY

To diagnose this you have to focus or possible movement between the disc brake mounts and the disc itself. Anything not within those "boundaries" doesn't factor.

I'd consider fork flex near the ends, hub bearing play, movement of the axle in the dropouts, etc.

For starters I'd pull the rim across toward a fork blade while watching for movement of the disc in the brake.

FWIW my prime suspect would be axle play, which is easy to confirm and fix or rule out.