Well, at least it's not the only one.
The analysis of all debut games in the new season of top 5 European league reigning winners from season 1998/2000 till now, shows that the only game that matches the same criteria was 2002/2003 Ligue 1 debut game Guingamp vs Lyon (3:3), the Lyonnaise side being the reigning champions of 2001/2002 season and conceding 3 goals in the debut match of the following season.
An interesting side fact is that this milestone has been achieved thanks to the future Chelsea FC's legend but then young Didier Drogba's late equalizer.