The tag is present in Lintian version 2.109.24. That is the most recent version we know about.
The version string in a historical changelog entry was not parsed correctly. Usually, that means it does not conform to policy.
It can also happen when a package changes from native to non-native (or the other way around). Historical entries are then in a nonconforming format.
As a side note, Lintian cannot tell whether a package changed from naive to non-native, or the other way around. It can only say whether the historical changelog entries comply with the current nativeness of a package.
Please refer to Debian Policy Manual section 5.6.12 for details.