Function reduce() and list of maps

The reduce() function loops on the list of lists that you've provided and the fourth list is one item longer than the others (i.e. the value 29). The function can only work correctly on lists that are the same length. To make it work you'll need to add an additional value 0 to all of the lists that are longer thus:

a!localVariables(
  local!sensor: {
    a!map(data: { 125, 92, 22, 63, 73, 28, 19, 0 }),
    a!map(data: { 35, 65, 92, 22, 63, 73, 28, 0 }),
    a!map(data: { 2, 35, 65, 22, 63, 73, 18, 29 }),
    a!map(data: { 9, 35, 65, 63, 35, 28, 19, 0 })
  },
  reduce(fn!sum, 0, local!sensor.data)
)

Alternatively, you can flatten the list and then simply use fn!sum() thus:

a!localVariables(
  local!sensor: {
    a!map(data: { 125, 92, 22, 63, 73, 28, 19 }),
    a!map(data: { 35, 65, 92, 22, 63, 73, 28 }),
    a!map(data: { 2, 35, 65, 22, 63, 73, 18, 29 }),
    a!map(data: { 9, 35, 65, 63, 35, 28, 19 })
  },
  fn!sum(
    a!flatten(local!sensor.data)
  )
)