As noted in the Math with Variables thread, the system will no longer “pretty print” array content.
Based on the logging that we added with the Math with Variables beta release, it doesn’t look like anyone is using this feature but we wanted to post the notice just in case someone was very rarely using the feature and their rule just hadn’t triggered yet!
As alluded to above, this was a very niche (undocumented) feature that existed in case someone used an HTTP Request and that returned a response with either simple numbers or text values. The new syntax/expression feature has native support for formatting arrays into strings and dropping this unique formatting case significantly simplifies things moving forward. 
Before:
$context.response.data //*only* if it was an array with just strings would it return a pretty printed value
After:
join($myArray, ', ') //you choose how it's formatted