$_GET and $_REQUEST empty when parentheses brackets

I have a request I am getting from an external site that looks something like this:-

https://www.myurl.com/?param1=1&param2=3oqaaaaySSGkkLh2HAInUuI5be9HdB&param3=asasauiisfjwof41fon9cdqpbjeqdxdm5m21rvzq&param4=(THISISTHEISSUE)

When I have the brackets ( and ) then $_GET and $_REQUEST are empty. If I encode the brackets as %28 and %29 then it works however there seems to be a redirect somewhere which is unencoding the brackets.

I am using PHP and WordPress. What could be causing these parameters to invalidate? Is there a way to allow it to accept them?