Just for my own edification, when (or with whom) does the classical period end? (No argument with your point.)
I gather that when we start talking about the "latest" old-school Epicureans who would have access to authentic texts and teachers we are probably talking about Lucian or Diogene of Oinoanda or Diogenes Laertius. I gather that their dates are approximate so I'm not sure which order to place them in.
After that period, it would appear to me that the continuity of the school was completely broken in terms of living teachers and readily available texts. Everyone after that general period would likely have been doing what we are doing - trying to reconstruct the full picture from relatively sparse remaining texts.
As time went by and fewer and fewer texts and teachers remained it would have become more and more tempting to narrow the focus onto the main surviving ethical texts and lose the context in which they were originally written.
Which eventually resulted in the modern phenomena of people who think that they can grasp all they need to know about Epicurus from the four short sentences of the Tetrapharmakon, which does not even mention pleasure or physics or canonics.