Same command broadcasts OK from command line, but not in a script - octave

Question

I'm relying on broadcasting in the following element-by-element multiplication of a 2x2x2 matrix by a 1x2 matrix (broadcasting means the smaller matrix is repeated as many times as necessary to match the size of the larger one - see end for more explanation). One command gives a "nonconformant arguments" error when in a script run from the octave command line. But exactly the same command runs fine if typed at the octave command line.
Occasionally, the script doesn't give the error, but I haven't worked out exactly what arrangement the Universe has to be in to avoid the error (see later) -- whatever, running it within a script is as robust as a cheap garden strimmer.
I give a workround below, but I'd like to understand what's behind this error.
Here's my script file mwe_bcast.m:
# Minimum working example of broadcasting an element-by-element multiplication.
# The last line errors when run from octave as a script,
# but not when typed at the octave command line.
clear
test = [0,1;2,3];
test(:,:,2) = 3 - test;
test .*= 2;
factors = [1,2;3,4];
test .*= factors(1,:)
But here's the error when run as a script:
>> mwe_bcast
error: mwe_bcast: .*=: nonconformant arguments (op1 is 2x2x2, op2 is 1x2)
error: called from
mwe_bcast at line 9 column 6
Nonetheless, when I then type the last line at the command line there is no error, and the following output shows that the broadcasting has worked as expected:
>> test .*= factors(1,:)
test =
ans(:,:,1) =
0 4
4 12
ans(:,:,2) =
6 8
2 0
Even weirder: I checked whether this is repeatable after restarting octave. It is and it isn't! Sometimes, if I run the script any amount of times straight after starting octave (after just a cd to the appropriate directory), it runs without errors. But as soon as I edit the script in any way (just changing a comment, and sometimes even just opening it in the octave editor), then re-run the script, I get the error. But, other times, I get the error the first time I run the script after restarting octave.
Workround: I get no error from the script if I don't use the abbreviated assignment (.*=) for this broadcast element-by-element multiplication, i.e. replace the last line with:
test = test .* factors(1,:);
However, just as weirdly, note that the earlier use of .*= in the script to multiply the same matrix by a scalar always works without an error, even though it also relies on broadcasting.
Explanation of broadcasting: Essentially, the last line multiplies the matrix
test(:,:,1) = 0,1
2,3
test(:,:,2) = 3,2
1,0
element-by-element by [1,2] which broadcasts as if it were the matrix:
factors(:,:,1) = 1,2
1,2
factors(:,:,2) = 1,2
1,2
System:
GNU Octave, version 5.2.0
Ubuntu 20.04 LTE
Linux 5.15