I know the kids created the green sun in a place where time isn't relevant so it can power the first guardians at all time but I thought the green sun is fading cause it's getting sucked into the black hole so jade is losing her 1st guardian powers?

Yeah she is, from her perspective, but again you just said it, Time isn’t relevant in the Furthest Ring. For most of the entirety of existence in every Universe that has ever been, with the apparent exception of the Universe C, the Green Sun is still an immutable constant. Homestuck’s story is precisely about the breaking of these constants, these rules, of tearing down the reality that’s always been there, of unravelling an unfair fate so that you may keep pushing on beyond whatever grand scheme the Alpha Timeline has in store for you.