Response header lookup provided by the AccessEvent are case-sensitive. The RFC for message headers says that the header names should be treated as case-insensitive [1]. Request headers are already treated as case-insensitive [2]. Response header lookup should be updated to be case-insensitive.

[1] https://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2
[2] https://github.com/qos-ch/logback/blob/v_1.2.3/logback-access/src/main/java/ch/qos/logback/access/spi/AccessEvent.java#L325