In case POS is in the supplier-State, is GST on reverse charge payable by recipient?

No, if POS is not in the State of the recipient, there is no occasion for the State of recipient to accept tax on reverse charge basis. In the absence of a specific provision in law, such supplies remain untaxed.

(ICAI FAQ PUBLICATION 06-09-2017 : Levy and collection of tax- FAQ NO. 36)