The Logback manual states
In PatternLayout, parenthesis can be used to group conversion
patterns. It follows that the ‘(‘ and ‘)’ carry special meaning and
need to be escaped if intended to be used as literals. The special
nature of parenthesis is further explained below.[…]
If you need to treat the parenthesis character as a literal, it needs
to be escaped by preceding each parenthesis with a backslash. As in,
\(%d{HH:mm:ss.SSS} [%thread]\).
You’ll need to escape the parenthesis with a \.
%d{yyyy-MM-dd HH:mm:ss.SSS} [%thread] %-5level %logger{36}.%M\(%line\) - %msg%n