Further investigation has revealed that the problem actually lies with the air braking value, where colliding with an object causes the acceleration to be marked as zero, which then causes the braking deceleration to apply, bringing the character to a halt.
Even more bizarre is that changing line 2177 in CharacterMovementComponent.cpp to:
const bool bZeroAcceleration = false;
fixes the problem and doesn’t seem to impact the braking deceleration at all when not touching any objects, contrary to what one would expect.