Does this mean that np.round(np.float32(5)) return a 64 bit upcasted int?
No. np.round() is an alias (which would be good to deprecate) for np.around(). No one has proposed changing np.around().
That would be really awkward for many reasons pandas frame size being bloated just by rounding for an example. Or numpy array size growing for no apparent reason
I am not really sure if I understand why LSP should hold in this case to be honest. Rounding is an operation specific for the number instance and not for the generic class.
The type of the return value is part of the type's interface, not the specific instance.