An important note for the exclude_line is that it gets applied only after the multiline has done. Still I would expect your above code to work except that some events probably contain a , at the end and indentation is not correct (not sure if that matters for LS)
The error you posted above comes from LS. So it seems you also do some json decoding on the LS side? What does your LS config look like?
Can you try to use the file output and see if in this file you get the expected output?