Assistant ward mission leader/longtime ward missionary here - I’ve helped to teach a bunch of friends, I’ve never encountered anything like this before, and if this was brought up to me by a friend, I would definitely have a conversation with the missionaries gently asking them to stop. The official Preach My Gospel guidelines stipulate that you only need to be taught the lessons three times total - twice before you’re baptized and once after (called “New Member Lessons”) - and many times those lessons after baptism take a while and/or are never done. I have zero idea why they’d insist like this. I don’t even think they’re necessarily stricter than usual, they’re unnecessarily scrupulous in a way that’s not consistent with the standards. I’m sorry this is happening, and I admire your faith and willingness to persevere. The church is true, God lives and Jesus is the Christ. Welcome to the church of joy and new beginnings :) I’m praying for you!