There are two problems:

• we're using URL#getQuery() instead of URI#getRawQuery() to extract the parameters, see https://bugs.java.com/bugdatabase/view_bug.do?bug_id=JDK-8214423
• we're not URL decoding the parameter name and value

This behavior seems to be pretty old, at least since https://github.com/xwiki/xwiki-platform/commit/1a340638ac6143092f5382799de6e9d0d7d7c3ee#diff-b5e7b083f35f9c5e987a7161945239df3c01452aa50a3ec48e8ee8f5afab1f91R185 .