Unity splash screen not appearing

Although we do not have a Pro account the Unity splash screen is not appearing.

We have published several games over the years using Unity. This is the first time that this has happened. The Unity splash screen appears when we are testing the game. It also appears in Win & Linux builds but it does not appear in the WebGL build.

The game in question is currently available online. We are aware of the license terms and have tried to make the Unity splash screen appear without success. We are a nonprofit org that publishes prosocial games for free.

We have a theory about what might be causing this but would rather speak directly with Unity to share further details. Please advise.