Try
1 2 3 rot3h¨⊂a
Enclosing a has the effect of turning it into a scalar, then you can succesively apply 1, 2, and 3 rot3h by scalar extension. (1 + 2 3 4 extends 1 such that it is added to all elements of 2 3 4)
Back to the original expression, when using outer product, the intuitive expectation would be that you get a matrix of matrices back. As the right argument is scalar and the expected result a vector of matrices, using each might be more natural.
Having said that, your original expression worked correctly and returned the same result in Dyalog APL.
Good example - I would report this bug to the GNU APL developers.