Reflections on a differential transmission line
First, in general, closely coupled differential pairs are somewhat above 50 ohms when each side of the pair is measured single ended (60 to 65 ohms is common).

A differential pair with each single ended signal at exactly half the differential impedance cannot (by definition) be coupled to each other, although many such scenarios exist.

That said, in a differential pair scenario, what matters is retaining the differential impedance; if the pair single ended signals move apart but retain the differential impedance, then there is no differential discontinuity.

If the differential impedance changes (regardless of the single ended impedance), there will be a discontinuity as given by \$\Gamma = \frac {Z_1 - Z_2} {Z_1 + Z_2} \$ where \$Z_1\$ and \$Z_2\$ are the differential impedances and \$\Gamma\$ is the reflection coefficient for voltage.