Rust warranty typically only covers perforation, not paint bubbling. That said, that looks bad and I would take it in and expect them to fix it on the grounds that if left alone, it will end up with perforation prior to the expiry of the rust warranty.

Looks like it originally missed the proper surface prep in that location.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue