What you are trying to do is currently not fully supported.
Our API documentation specifies the following for operations related to allocations:
Resources related to the allocations in the API. This allocates a set stock item to a specific order. To allocate stock to an order, the item must be added as a line item within that order and not be allocated already.
I can confirm that POST and DELETE requests for allocations work as expected.
I can also confirm that PUT requests are not fully functional at the moment, which would explain the 200 responses.