Google Sheet custom Dialog - Splash screen not displaying body text

I am creating a splash screen in Google sheets using a Custom Dialog with the following code:

let htmlOutput = HtmlService
   .createHtmlOutput('<p><font size="4" face="arial">' + msgTextHTML + '</font></p>')
   .setWidth(width)
   .setHeight(height);
ui.showModalDialog(htmlOutput, titleText);

When the code executes the custom dialog is displayed but only with the title text, the body text is only displayed after a significant delay. As far as I can tell, it is waiting until after the script has completed before displaying the body text and is thus not a very good splash screen. Is there any way I can force it to display the body text immediately?