Github user mikaelstaldal commented on the issue:
https://github.com/apache/logging-log4j2/pull/110
@mdvorak Resolving `${}` expressions during serialization is necessary to
support dynamic lookups which depend on the current log event, such as
[ContextMapLookup](https://logging.apache.org/log4j/2.x/manual/lookups.html#ContextMapLookup).---
