Most if not all sprinters make contact with the midfoot. Some make contact with the heel. I really don't think it's an issue one way or the other. In fact what you're seeing could actually be a consequence of good running mechanics….if the athlete has the toe up (in a neutral position rather than plantar flexed) late in the swing phase just prior to ground contact they'll make ground contact slightly more under the hips (less braking forces) than if they landed in a more plantarflexed position. If they make contact in this neutral position it is unlikely that they will be able to plantarflex the ankle until toeoff because of the tremendous load and the relative weakness of the gastoc / soleus….thus giving the appearance that they are making contact with their heels (when in some cases they are likely close but not fully in contact). What I would not like to see is a heel first landing because I don't think it loads the gastroc / soleus as well as a midfoot contact would.