Focus tracking on 5D4

[carloferraro]

I was trying today a slider on the 5D4 in video, using -as usual- AF tracking mode, 

for my surprise, going back and forth on the slider at moderate tele the camera does not focus.

Never tried this before, my question is: tracking  AF only works horizontally?

Thanks

[Waddizzle]

What were you tracking?  Faces?

[carloferraro]

A flower, dollying in/out.

I do not do faces but the tracking AF  works fairly fine with animals and long teles.