rundll32.exe (this is a system file of Windows) is what’s used to run the global injection service of Special K. That BitDefender was triggered on the SysWOW64\rundll32.exe means it was triggered by SpecialK32.dll which was loaded by rundll32.exe to run the 32-bit global injector.
No need for you to do anything, really. I’ll report the false positive to BitDefender.