I would advise you the following:
1. Since the builder registered the agreement at a belated stage in 2017, the possession was handed over to you at in that year itself. Therefore you are not liable to pay maintenance before the registration of 2017;
2. If the builder is still demanding so, you can send him as well the society a legal notice;
3. Can I ask when did the OC of the building come?
4. Relevance is placed on the case decided by the National Consumer Disputes Redressal Commission in the matter of Runwal Developers Pvt. Ltd. and Another Vs. Dinesh Hegde and Another, whereby it was inter alia observed that the flat purchaser shall be required to pay maintenance charges only when the possession of the flat has been given to him.
5. Similarly in the case of, Hermione Mary Salazar vs Mr. Anthony H. Silva, the State Consumer Disputes Redressal Commission reiterated the importance of section 6 of MOFA and protected the rights of the allottee regarding the payment of maintenance.
6. Thus, it is not only contractually as well as legally, the maintenance charges in respect of the said Flat are to be paid by the Developer. Even The Maharashtra Ownership Flat Act, 1963 casts the obligation on the Developer to pay the outgoings in respect of a Flat/unit until handing over the same to the purchaser.