The charge indicator should be thought of as a rough gauge of the fact that the battery is receiving a decent amount of power.
In low-light conditions, the battery can actually be receiving power, but not enough to cause the charge indicator to flash. For example, the threshold at which the V15’s charge indicator is activated is approximately 50mA, but the circuitry of the battery is not designed such that this is a hard cutoff. If your battery lights up briefly then stops, that could be due to either the fact that your solar panels’ output isn’t very consistent, or just that that the power to the battery happens to be right around that threshold. If the charge indicator is lighting up continuously, you can expect that the battery is consistently receiving a good amount of power.