For the question, I think @JonMikelV has the right answer.
To clarify further, Duplicati works with “backup sets” or “snapshots” which is “a collection of files and folders as it looked at a specific point in time”.
Each backup makes a “snapshot” of your files. This snapshot includes all files currently on disk. For your example, each snapshot will include both test1.txt and test2.txt.
When you delete older versions, you delete snapshots. For test1.txt that means you will loose the ability to restore the older versions of that file. For test2.txt you will not loose anything.
If you delete test2.txt from disk, then you will be able to recover it until 180 days have passed, and the last snapshot with the file is deleted.