I don't know if it has anything to do with it but I now see that the repo which is cloned and save in the cache is actually a different branch than the one which is pulled by the agent.

From the log file:
repository = https://gitlab.com/data_science_team/PROJECT_NAME branch = MYBRANCH
SOMENUMBER IP### DEBUG
warning: redirecting to https://gitlab.com/data_science_team/PROJECT_NAME.git/