Hi @rajiv kumar ,
Have you made some customization to the specific page? If possible, please provide more information such as gif about this.
As far as I know, there is a delay in application customizer extension by default, which is caused by the loading sequence of modern pages.
If you use any JavaScript or Jquery to customize, this is technically by design how the JavaScript files are loaded, as a workaround, insert the CSS directly in your SPFx extension.
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