Almost always a stale pairing on this phone. A Chorus leader is never paired — the page talks to it directly. If it was once added from Bluetooth settings, the phone keeps a key the leader does not have, and every connection after that is dropped the instant it opens.
Settings → Connected devices → Chorus Leader → Forget, then come back and press connect. Do not pair it again.
If it is not in that list, the leader is probably out of range or off.