If you use the return rune on a new realm without activating any whispers, you will be taken to Sacrament but the gate bridge will be impassable (back to the glades) and the cutscene will not trigger at the rookery.
This essentially means you’re stuck in Sacrament without being able to progress the game at all.
You can leave Sacrament via the other route into the Glades. Head towards the gate/drawbridge, then north / northwest and climb up the vine to get up onto a tower of sorts that will let you climb down into the Glades.
I haven’t even been that route in my main world! Thank you for clarifying (tho I still feel return rune shouldn’t work until you actually unlock Sacrament via normal means).