ASCII statistics logging is spitting out a json-like header line on every iteration. It looks like:
,"wiredTiger":{Sep 11 14:43:20 0 . LSM: application work units currently queued Sep 11 14:43:20 0 . LSM: merge work units currently queued Sep 11 14:43:20 0 . LSM: rows merged in an LSM tree Sep 11 14:43:20 0 . LSM: sleep for LSM checkpoint throttle
This appears to be a problem in statlog_print_table_name not checking if we want json output and was introduced a couple months ago as part of WT-3140.