Is it possible to have a false negative rapid HIV test well after window period (9 months after last possible exposure)? If so, why?

Unlikely. When sufficient time has elapsed after one's last potential exposure, HIV tests are usually reliably positive if one is in fact infected. One has to be aware that the usual test looks for hiv-1, but there is also an hiv-2 which is far less common but could be in you and the standard test would not recognize it.