I now figured out what the problem was. I selected 9 triplets and deleted the brackets and then made them into 6:4. This resulted in drifting. However if I only choose the first 6 triplets and do the same then everything is where it should be at the end. The result makes sense: 6 notes as 6:4 and 3 notes as 3:2.
Thank you very much for your support!!!