Hi Raoul,
thank you for clarification.
Oracle may uses a different undo allocation algorithm (check previous posted blog post) after changing the data file attributes to autoextend on. Be aware if maxsize is currently greater than the actual size then the algorithm for calculating the tuned undo is based on free space again.
Depending on the used algorithm you may want to troubleshoot it further, but currently it is hard to point you in the right direction as we don't know the exact state of your database.
By the way you can also see that Oracle tries to steal unexpired extents "heavily" after restart only and that the transaction count went up very high (not quite sure if that is a result of the V$UNDO_STAT "issue" or if this is real, but you can verify this by generating several AWR reports and comparing the statistics).
Regards
Stefan