Tha Rambam actually holds that Nismaatu Halevavos and Lo Shimshu Kol Tzarkan doesn’t mean that they forgot Halachos. It means that they didn’t receive the entire mindset and judgement. When new issues came up they didn’t necessarily see things the way their Rebbe would have. This caused Machlokes.
This is hard to square with many basic Machloksim, including the first one. There are Gemaros that speak openly about a Talmid not hearing correctly. Perhaps the Shomer Yisroel made sure those instances were rooted out. Also, the Rambam might only be addressing that Memra about the main cause of Machlokes. Or, perhaps each case will need its own Pshat to explain what was before the Machlokes and what they were taught.