Unfortunately there is no fix, this bug started occurring more commonly around Update #5 (November 2023) and hasn't been fixed since.
Some other people observed that him not spawning may be tied to his pathfinding once the conversation with Ethel is over, so as a workaround they recommend triggering the cutscene by teleporting to Johl's left side so he can properly despawn offscreen. But I personally cannot confirm whether that works or not.
