'Conspiracy' plot hole

[Bingo]

This problem bothered me too. The only apparent solution is that Sam's drying up did not include reporting the conversation from The Shadow of the Past. At the end of the chapter, we see this:

‘if you
really care about me, you will keep that dead secret. See? If you don’t,
if you even breathe a word of what you’ve heard here, then I hope
Gandalf will turn you into a spotted toad and fill the garden full of
grass-snakes.’
Sam fell on his knees, trembling. ‘Get up, Sam!’ said Gandalf. ‘I
have thought of something better than that. Something to shut your
mouth, and punish you properly for listening. You shall go away with
Mr. Frodo!’

If Sam wanted to interpret this interaction narrowly, he could reason that his punishment for reporting the conversation was to be sent away with Frodo, which was already happening anyway.

According to Merry:

‘Here’s our collector of information! And he
collected a lot, I can tell you, before he was finally caught. After
which, I may say, he seemed to regard himself as on parole, and
dried up.’

This is at least consistent with the theory that it was the collecting that stopped, not the reporting of already collected information.

We also know from the conversation with Gildor and the Council of Elrond that Sam was not above eavesdropping even after being caught.