Column entries are displayed as numbers / IDs in Timeline plugin

Hey all!

I am using a modified version of the project tracker template by Sea Table. I have a table showing projects and a connected table showing milestones for the projects. One project has 1 or more milestones.
Now, I want to display the milestones (they have deadlines) in the Timeline plugin. The illustration is as I want it. However, the project names are not displayed as text but as a number, which I consider an ID or similar.
The screenshots below show my problem. The goal would be to actually display the project names instead of IDs in the Timeline plugin.

Thanks in advance!