The user is using Addressables to load a Timeline Instance, which is a prefab that contains a Playable Director component. This component has a reference to the Timeline Asset.

The Timeline Asset contains Signal Emitters, which have references to the Signal Assets. There is another GameObject in the scene that has a Signal Receiver component, which also has references to the Signal Assets. The issue is that these Signal Asset references aren't considered equivalent.

See this blog post for more information about Timeline Signals: https://blogs.unity3d.com/2019/05/21/how-to-use-timeline-signals/.

One workaround is to manually set the Signal Asset references on the Signal Emitters. The Signal Asset references are stored on a GameObject in the scene.

In the example below, the references are stored on the GameObject containing the AddressablesInstantiator script component.

SignalTrack signalTrack = (SignalTrack)asset.GetRootTrack(3);
List<IMarker> markers = signalTrack.GetMarkers().ToList();

AddressablesInstantiator instantiator = FindObjectOfType<AddressablesInstantiator>();

for(int i = 0; i < markers.Count; i++)
{
SignalEmitter emitter = (SignalEmitter)markers[i];
emitter.asset = instantiator.signalAssets[i];
}

Another workaround is to put the Timeline Instance prefab in the same scene as the GameObject with the Signal Receiver component. At this point the timeline signals will function as expected when entering Play Mode or running a build.

To continue using Addressables to load the objects, create a new scene that acts as a bootstrap scene. Then use Addressables.LoadSceneAsync to load the scene containing the Timeline Instance and the GameObject with the Signal Receiver component.