Nice work. I haven't seen this in the samples that I have. You may want to edit this question to have a a question like "How can I remove these slashes from my ACS logs?" and then answer it. That way we can upvote it and it can be marked as answered!
i've added the following line to my props.conf to clean that event data from ACS logs. same trick i used from an old version of the ISE app. i'm fairly certain it's a bug / enhancement on the cisco side for both products.