Unless there's been a bug introduced, you should be able to acquire all the talk topics by the time your knowledge caps out. Is it possible you have clothing artificially inflating your knowledge and you aren't actually at the cap? I don't *think* clothing bonuses count towards actually learning topics.
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Actually I've encountered this a few times especially while using cheats sometimes the game will not prompt you to learn the talk topic and it will lock you out of learning it entirely.
I highly doubt it was intended to do so without without cheat mode on so it's always best to pick tv talk topic first