Tags disappear from the graph view on the right.

I could reproduce the problem. It happens when creating a new tag based on another tag and removing the old tag.

Steps to reproduce in SourceTree:
1. Create a new git repository
2. Create a file (touch x) and commit that
3. Create a new tag for the working copy parent
4. Create a new tag and enter the previous tag name in "specified commit"
5. Delete the old tag
6. No tags are shown anymore in the graph view

Tags are shown correctly in the "Tags" section on the left. But they are not shown in the graph view to the right.

Interestingly, it does not happen when creating the new tag in the commandline with "git tag NEW OLD" - but SourceTree copies more info from the old tag to the new one than this command which is good.