From this link the form should be restored only after the submit has finished.

On submission of a form bound with a phx-submit event:

1. The form’s inputs are set to readonly
2. Any submit button on the form is disabled
3. The form receives the "phx-submit-loading" class

On completion of server processing of the phx-submit event:

1. The submitted form is reactivated and loses the "phx-submit-loading" class
2. The last input with focus is restored (unless another input has received focus)
3. Updates are patched to the DOM as usual

What does the log show?
Are you doing something heavy in the server that would be better handled by async tasks?
Have you tried enabling livesocket debug and checking the console for errors? Maybe a timeout?

window.liveSocket = liveSocket

// in the browser's web console
liveSocket.enableDebug()

Without more info is hard to know for sure, but if it is some kind of timeout due to heavy work my suggestion would be to make everything as light as possible for the user and do the heavy work outside the main codepath.