Why would call to async_add_entities() not add entity?

I’m a custom component developer and some users have reported that entities aren’t getting added except at restarts as per New device_trackers only being added after reload of intergration · Issue #134 · amosyuen/ha-tplink-deco · GitHub. I’m not able to repro this myself.

Based on the logs provided, this code is being executed so async_add_entities() should be called with the new entity.

Anybody know if there is something that would prevent the new entities from being added?