Open MercurialEclipse history for MANIFEST.MF file.
Check the tags RELEASE_1.7.0 and RELEASE_1.5.0: they are shown at changesets which are submitted to NON default branch.
Check same tags for the root history: you will see that tags are placed properly on the default branch.
In general, the tags for folders or files in the history view are shown incorrect if there closest file/folder revision was made NOT on the tag's branch.
Please note, that this bug affects only file/folder history - root history uses tags as provided by Mercurial, so there is no issue.
Tags should match the correct branch in the file/folder history.
I have a fix.