New code period with Git plugin

The SonarQube instance of a large project has recently been upgraded from SonarQube 7.6 to 8.4. The Git plugin was disabled (through sonar.scm.disabled property) for this project as the initial scan was taking hours, so I was using projectVersion to compute the new code period. It still works well in SQ8, but I wanted to test the new features of SQ8.
I analyzed the current production version with Git plugin activated, then analyzed the development branch: new lines of code stays at 0, although I see the right number of files in analysis log (SCM Publisher 50 source file to be analyzed). But if I create a new commit on the development branch and analyze again, then the code from this commit is flagged as new code. I would have thought that new code would be computed from a git diff commit_id_of_new_code_period_baseline, but it seems to be based on timestamps. Am I doing anything wrong ?

SonarQube 8.4.2 Community, Git plugin 1.12
Maven code scanner