In this paper we report on limits related to the development of optical fibers with glass subwavelength inclusions incorporated into the core. We present the fabrication of a photonic crystal fiber made of an in-house developed silicate NC21 glass with a subwavelength-size high refractive index inclusion in the core made of lead silicate SF6 glass. The core has a diameter of 2 µm, while the diameter of the inclusion varies from 0 to 800 nm. Using energy-dispersive X-ray spectroscopy technique we show a dramatic change in the inclusion profile and its composition caused by a non-uniform diffusion of chemical molecules during the stack-and-draw fiber fabrication process. Therefore, the effective refractive index and the material dispersion of the final fiber are significantly different than for bulk glasses, which leads to an alteration of optical properties of the final fiber. A unique non-monotonic characteristic of the effective material dispersion is used to reproduce the fiber dispersion characteristic.