A strange thing I noticed is My404Page was not working properly on Firefox, I then tested it on IE and it worked.

When I browse my website it shows the HTML tags on the page.

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When I test the site using https://dewani.net/123.aspx My custom 404 page showed up correctly

When I test the site using https://dewani.net/123 My custom 404 page showed up HTML tags.

When I digging and goggling I found it