It appears that URLs of triggers in frontend are urlencoded twice. Trigger URLs in emails send by actions are not affected.

The configured URL:
https://www.example.com/context/?foo=1&bar=2&baz=3

gets evaluated to:
'URL',"https:\/\/www.example.com\/context\/?foo=1&bar=2&baz=3"

and finally results in:
https://www.example.com/context/?foo=1&bar=2&baz=3

I'm quiet sure that there already exists a ticket for that, but I didn't found it.