You would like to use the "url.query:decode_base64" in exception pages or notify pages but found that when the page is displayed out, it adds extra garbled messages in the page.
After SGOS 188.8.131.52 and later, this happens because there is an anti-phishing feature that was added causing the extra garbled text to display out.
Versions before SGOS 184.108.40.206 doesn't see this issue because the anti-phishing feature was not added yet.
Use a new substitution variable CPL as per below :
With this new variable the extra garbled text would be gone.