Confirmed.  I can probably understand why as well.  Since the variable doesn't exist, all trailing dimensions are probably set to 1 so the implied size is 0x0x1.  The ':' indexing operator selects that last dimension, but it has size of one versus a size of three.  This squares with the error message issued

nonconformant arguments (op1 is 0x0x1, op2 is 2x3)

I'm guessing that this would be relatively hard to solve since it involves adding a very special corner case.

The following works in matlab (2016b):

>> clear all
>> q(1,3:4,:) = ones(1, 2, 3)

q(:,:,1) =

     0     0     1     1


q(:,:,2) =

     0     0     1     1


q(:,:,3) =

     0     0     1     1

while in Octave 4.3+ it produces an error

>> clear all
>> q(1,3:4,:) = ones(1, 2, 3)
error: =: nonconformant arguments (op1 is 0x0x1, op2 is 2x3)