Something similar happened to me. Enemy was within a stasis grenade area of effect. One of my terminators engaged it in melee from outside the area, pushed it and moved to pursue, suddenly that terminator had movement 42

I've completed about half of the first campaign and it's happened to two of my units so far. They level up but no new skill shows up when I click on them neither in the force selection screen nor later in the mission itself.

Astra Militarum and Chaos Space Marines