"Column" fields on release timeline report are partially under the milestone marker when two rows of fields are attached to the marker. When adding Release Name, and Release Theme for example, the Release Theme is placed under the Release Name, and it is partially underneath the milestone marker.
This is a known issue with the report timelines that some fields will not appear properly on the timeline. This exists for the Release Theme field, as well as other description-related fields due to the ranging amount of text on these types of fields.
While we understand that there may be some situations where it may be helpful to provide the Release Theme on the timeline report, this field is best suited for List or Pivot table reports - as there are many complex issues involved in trying to better handle the formatting in this use case. Upon a closer look, due to the complexity of the issue and our current priorities at hand, this is not an item which we are able to work on in the near-term and will mark as unlikely to implement. I hope you can understand.