...

When the consumer does not own a partition anymore, the lag metric will be removed.

Notice that the per partition lag metrics have an average value reported while the max log across all the partitions only has a max value reported. This is because the average value of max lag from all the partitions may be an average from different partitions at different times, so it is not very useful. But the average value of per partition lag at different times is more meaningful.

Compatibility, Deprecation, and Migration Plan

...