<:LF:> is a special formatting token that we came up with. Since we treat each line of STDOUT as its own "data item", if there is ever a single data item (such as an error message) that has multiple lines, we replace those lines with <:LF:> as a substitute. Those tokens are then later on translated back into new lines by our formatter code. Its the same formatter code for all languages, so its interesting that it makes it all the way to the UI, but only for Java.

The custom junit formatter might be html escaping the <:LF:> tag. I've added a Github issue.

Okay, this is a bug with JUnit. We'll open up an issue on github.