I found the race condition in the Logback:
So, when we have multi-threaded appender, we end up with the issue writing to the same StringBuilder concurrently.
See this consequent commits in Logback:
The problem with the last one that it isn't backported to 1.1.x.
Since the fix has been done in master, please, consider to backport it into 1.1.x.
See https://github.com/spring-projects/spring-boot/issues/8354 for more info.