I found a bug on Arena with Cogwork Progenitor and Assimilation Aegis.
Do you remember how ******* Taker used to loop with itself, allowing an arbitrary number of Enters The Battlefield (ETB) effects ? It seems to work as proof, if needed, that "When [...] ETB, exile target creature until [...] leaves the battlefield" will loop as long as you choose to target [...].
I used Cogwork's ability to seek an Aegis and perpetually give it the creature type and the Gnome subtype (cf screenshot).
Then I cast it, then the ETB triggers and I target the Aegis itself (cf screenshot). Then it resolves, Aegis exiles itself "until it leaves the battlefield" (so should immediatly re-enter the battlefield) but stays in exile (cf screenshot).

Thanks for reading !
Seb