That’s a GameMaker bug/feature - this happens with any loading screen extension, for whatever reason.
Viewing post in HTML5 loading screen for GameMaker comments
I think what’s happening is that GameMaker now shows your splash screen on its own until it gets the loading screen extension loaded, so that’s not something that you can address in the loading screen code itself.
If you don’t want to bother YYG about this, you could fork the HTML5 runtime, find where that’s happening (commit history should be descriptive), and change it back to how it was before.