thank you for your answer.
Yes, the code is run on exactly the same tree and both when using the Print() function after opening the root file and in the TBrowser, the new branch shows up. Hence the confusion…
That worked. Opening the files with the option ‘update’ afterwards also worked. I went back to trying my original code, without a problem, also tried separately in a clean environment. To be honest, I have no idea what happened and I feel like I have no way of reproducing the error.