Ultimate Tic-Tac-Toe : How is the smaller board chosen each turn?

I would like to know how the smaller board is chosen each turn.

I use an empty list in my code, and append each valid_action_count to this list each turn.
Then I use the debug print to determine how many and what actions are available each turn.
What I see is that on turn 1, I have 91 ( 9x9) actions to choose from. Excellent.
However on my second turn, I have only 9 actions in the valid_action_count list.

Am I correct in observing that after the first turn I am forced to place my mark in a random smaller board rather than one of my own choosing?

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