I think there is an inconsitency, but generally most ISMs will come and give a greeting. As it is not a published benefit, I guess inconsitency will always be the case.
I have only flown in J as a GO member, but each time I was greeted with a "welcome back...etc..." while being handed a menu. In J the greeting can be less recognisable and exclusive, becuase the FAs generally greet everyone in J at the beginning of service, usually with the hand out of menus. Regular passengers usually get a polite "welcome aboard Mr./Miss...here is your menu etc..." whilst MPOs get a "welcome back Mr./Miss... here is your menu etc..." or on the lines of that. At least that is my experience. I have had the ISM come greet me personally twice in J on top of this.
In Y I think the head of Y cabin or ISM or both can come and say greeting. Therefore it may be that if both think the other will do it, it may not happen. Some ISMs perhaps only wish to deal with premium passengers leaving head of Y cabin to deal with Y passengers. The head of Y cabin may not always come and say hello because ISM might do it... therefore no greeting. Just a thought.
Despite the inconsitency, most of the time GO/DMs and even occasionally SLs get a greeting. This is much better than other airlines. BA for example doesn't greet anyone personally, so we are lucky that we have this personalized greeting on CX in the first place!