To quote the fellow in "The Wonder Above":
There was a time, not long ago, when this constellation was absent from the night sky.
Implying that it disappeared in the first place. Potentially, even those this is conjecture, Bard's constellation disappeared not too long after he left with the relic and the shrines, signalling he had left the physical plane of Runterra. The constellation has become apparent again as a signal of his impending return. Perhaps the star signs "appear" more strongly and "disappear" all together in correspondence to when things art happening in regards to them; Bard just left Runeterra; the constellation sparkles like a firework display, and then disappears afterwards, kinda like a natural omen.
Also by appear and disappear I don't mean stars just up and vanishing, but more like the constellations being harder to spot and the stars that make them up being obstructed or dim.