When a boss is nearby the Prime will attempt to suck it into its vortex, but will often fail to do so completely, leaving the boss in stasis without consuming the internal reservoir dry ever. As you can see, there's a dragon really far away and it is being held in stasis by the distant Prime. These bosses do not seem to quickly consume the Fountain's reservoir unless they are contained at the very middle of the vortex, if they are not, it just consumes a small amount.