We present experimental results showing alternating lasing and non-lasing regions for the short-wavelength longitudinal mode in a GaAs-based 850 nm coupled-cavity vertical-cavity surface-emitting laser (CC-VCSEL). These regions are situated between the laser threshold and roll-off for this mode. The analyzed structure consists of two identical AlGaAs cavities with GaAs quantum wells, separated with 11.5 pairs of middle DBR. The current apertures are realized by ion-implantation for the top cavity and selective oxidation for the bottom cavity. We then perform fully-vectorial three-dimensional cold-cavity optical simulations to theoretically investigate optical density radial and on-optical-axis profiles of the first order transverse modes corresponding to the two longitudinal modes. We show that the short-wavelength fundamental mode λS-LP01 is subject to periodic changes of its optical field distribution when changing the oxide aperture radius, which can lead to weaker resonance of the short-wavelength LP01 mode within the coupled cavity structure.