A model version is Unverified when you marked one or more issues as Fixed and there are no newer versions of the model to verify the fixes.
For example, if the Amount measure has an issue and you declare it as Fixed, the following version of the same model should not have the same issue. If it does, then the changes applied to the Amount measure did not solve the issue, which is reported again in the new version.
In other words, DAX Optimizer does not trust an issue as Fixed in a new version of the model. DAX Optimizer skips the analysis of the Ignored issues if the measure did not change in the new version, but analyzes all the measures previously marked as Fixed to verify the fix.