I would guess that the residual strength is a result of fatigue failure of the matrix allowing the fibers to straighten and align more closely with the off axis load path.

You can realize that the strength is mainly carried out by the fibers and that the contribution of the matrix to the strength is almost negligible . That is to say that, at 45 degree , the modulus is matrix-dominated, that is not the case of residual strength.

The behavior you are reporting is quite general and and occurs almost always during fatigue test in any kind of composites (even if  large areas with diffuse damage are detected with some techniques). In fact, the residual strength remains almost unchanged during the fatigue test until the number of cycles reaches a critical value where the residual stress undergoes a sudden drop.

Please see the following papers to appreciate the complexity of residual strength's degradation phenomena in several composites.

  Alberto D’Amore, Luigi Grassia "Constitutive law describing the strength degradation kinetics of fibre-reinforced composites subjected to constant amplitude cyclic loading" Mechanics of Time-Dependent Materials Volume 20, Issue 1, February 2016 ISSN: 1385-2000 (Print) 1573-2738 (Online)

 Alberto D’Amore , Massimiliano Giorgio, Luigi Grassia “Modeling the residual strength of carbon fiber reinforced composites subjected to cyclic loading” International Journal of Fatigue 78 (2015) 31–37

I agree with Pr. Alberto D'Amore. And thank you for sharing your publications.