Where I live, the " 've" is not "understood" or assimilated: it isn't there in any form. I would bet money that the majority of people here who say, "I seen that movie" [gritted teeth e] have no idea that what they're saying is incorrect English. And for the minority that DO know it's incorrect, most of them don't care.

I cannot, of course, say with certainty that there've been no times when a speaker has elided the 've into the next word and I didn't hear it, but I will say that there have been many times when the 've IS audible.