An initrd.gz generated under current64 with multilib is much bigger (5.8 MB vs 4.8 MB) than one generated under current64 without multilib. Some research shows that the "with" /lib contains some 32-bit libraries, whereas the "without" /lib does not. For the remainder "with" and "without" are identical. The 32-bit libraries have no meaning in the initrd.
/sbin/mkinitrd contains 2 "find /lib /lib64 ... | cp ... $SOURCE_TREE/" constructs. As i?86 and s390 have their libraries in /lib and x86_64 in /lib64 this is a clever way to not have to sense the machine type. But with x86_64/multilib things turn out to be too clever. The attached patch resolves the problem.