Is part of an output of secure PRNG also cryptographically secure?
Yes. Although the bit values are not identical per se (even given the same seed) you should use any of the 192 bits to form a 128 bit key - as long as you don't reuse any bits of course.

If the 128 bits are not secure, then the 192 bits should also not be secure. AES keys consist of bits that are entirely random and thus unrelated, so generating a 192 bit key and then using the leftmost 128 bits should be OK.