Fixed!
Actually, the same problem occurs with most interpolating geometric transformations in the PJSR (translation, resample, rotation, etc.). The cause of this problem is NOT in my code, but in M$'s particular interpretation of the C++ language... better stop writing at this point
Suffice it to say that the same code works perfectly when compiled with g++ (FreeBSD, Linux and Mac OS X), just because g++ is a good standards-compliant compiler.
Anyway, the problem is now fixed. I'll publish an update to the Windows versions of PixInsight Core ASAP.