If lightspeed is constant, and the distance between origin and destination increases due to spontaneous eruption of space between them, then lightspeed would appear to slow to observers keeping an eye on it. But it's not space that erupts - it's spacetime.

Since speed is a function of both time and space, lightspeed appears constant because both time and space are actually one thing. Although the light now has more space to traverse, it also has more time to traverse it. The speed of light remains constant.