There is no obvious mismatch between the LHS and RHS shapes. Otherwise, the code wouldn’t have worked with hybridize() commented out; right?
By the way, searching on the internet, it does look like others have run into this error in the past as well, though not in the context of HybridBlocks.
Yes, this is documented in the symbol API that implicit broadcasting is not supported.
Basically, in your hybrid_forward method you either want to use parts of the API that are the same between the ndarray and symbol modules or conditionalize based on whether the block is active (or the value of F).
Another thing to avoid is the use of the [] slicing notation.