This degeneracy arises from symmetries in the caustic structure. The caustic is defined in lens-centered coordinates, such that the impact parameter, u, can take positive and negative values.

For the binary-lens case shown below, the lens-source relative trajectory means that the source intersects the caustic in two places where the magnification is identical, producing an identical lightcurve. These points occur at u and -u.

In point-source, point-lens models, the caustic is a point. Since this is symmetrical from all directions, the models do not have an α parameter, but the same impact parameter degeneracy exists.

Caustic structure with source trajectory intersecting with negative impact parameter
Caustic structure with source trajectory intersecting with negative impact parameter [R.A.Street]
Caustic structure with source trajectory intersecting with positive impact parameter.
Caustic structure with source trajectory intersecting with positive impact parameter [R.A.Street]
Lightcurve of source trajectory intersecting caustic with negative impact parameter
Lightcurve of source trajectory intersecting caustic with negative impact parameter [R.A.Street]
Lightcurve of Caustic source trajectory intersecting caustic with positive impact parameter
Lightcurve of source trajectory intersecting caustic with positive impact parameter [R.A.Street]

The same phenomena has been observered in events with a measureable annual parallax signature, such as Dominik et al.(2018).
Parallax affects both single and multiple lens models by introducing a skew (or in extreme cases a sinusoidal curve) into the lightcurve on top of the ‘static’ microlensing signature, as illustrated below.

Caustic structures for relative source trajectories with and without parallax
Caustic structures (red) for relative source trajectories without (black) parallax, and with parallax with u00 positive (magenta} and negative [R.A.Street]
Lightcurves for binary models with and without parallax
Lightcurves for binary models without (black) parallax, and with parallax with u0 positive (magenta} and negative (green) [R.A.Street]

The caustic for a given single or binary model remains the same shape; the effect of annual parallax is to introduce a curvature into the relative lens-source trajectory. This is the result of the observer moving during the event.
There is still a u0 degeneracy if this curve trajectory intersects the caustic at the same angle α due to the symmetry of the caustic.

Referencs

Dominik et al.(2018), MNRAS, 484, 5608.