Hello,
Yes, you are very well correct that you have to pay GST at the time of issue of invoice no matter you receive the payment later on or may not receive the payment at all.
But the law is such, according to the law the government will recover its taxes when its due, its your fault if you cannot/ do not recover your dues from the party concerned why should the government suffer from it. The law was the same even at the time of service tax/ vat, it is nothing new. (please note this is the government's view and hence the law is framed accordingly)
Trust this clarifies your query.
Feel free to call / get back in case of further clarifications.
Thanking You.
Regards,
Rohit R Sharma
BCOM, FCA, LLB, CERT. FAFP