urlencode_deep function and wrong URL

This seems like a bug with urlencode_deep function:

print_r( urlencode_deep( array( 'width' => '80%' ) ) );

The result of that is:

Array
(
    [width] => 80%25
)

Notice the extra 25 in the parameter.

The why:
I’m using add_query_arg function which in turn uses urlencode_deep function on all the parameters from the URI passed to the function. The problem is urlencode_deep function messes up one of parameters sent, so this:

https://site.com/?width=80%

Which is valid URL, inside the add_query_arg function becomes:

Array
(
    [width] => 80%25
)

Any idea why?