I created a use case but saw it not only affects the AOP process but any file you want to open in the Modal Dialog. With previous versions of APEX, this didn't happen, but with APEX 21.1 the Modal Dialog keeps showing the spinning icon.
On the modal page behind the "View File" button, there's a before header process which gets a BLOB (PDF) from the database and displays it inline.
On the modal page behind the "AOP Modal" button, there's a before header AOP process which generates a PDF of a classic report on the fly.
After waiting 2000 milliseconds (2 seconds) it will add the js-dialogReady CSS class, which tells the icon to hide.
Hope it will help you in case you need to remove the spinning icon from your modal dialog.