Strictly speaking, the transfer of rights cannot be completed without registration, but other terms in the contract, such as breach of contract and payment, are valid if there are other special provisions. So we can't say that the whole contract is invalid.
Look at the second paragraph of Article 15 of the Fact Rules of the Patent Law again.
..... shall be filed with the administrative department for patent in the State Council within 3 months from the date of entry into force of the contract.
In other words, the absence of filing (registration in the patent law) does not affect the effectiveness of the whole contract. When applying, it is decided whether the patent subject is changed, that is, whether the patent is transferred.