My confusion was regarding inner padding of data passed to the array<vec3f>. It is now clear, that in order for it to work correctly, the data should be padded:

[
  1, 2, 3, 
  4, 5, 6, 
  7, 8, 9
] -> 
[
  1, 2, 3, 0, 
  4, 5, 6, 0,
  7, 8, 9, 0
]

My confusion was regarding padding of data passed to the array<vec3f>. It is now clear, that in order for it to work correctly, the inbound data should be explicitly padded, thus:

[
  1, 2, 3, 
  4, 5, 6, 
  7, 8, 9
]

becomes

[
  1, 2, 3, 0, 
  4, 5, 6, 0,
  7, 8, 9, 0
]

In order for it to work correctly.