Positive Test Results Only Once

I started having MG symptoms in 2021. However, all my test results were negative for an entire year. Then, in 2022, I tested strongly positive for MuSK. Now, in 2023, my test results are negative again, but I still have symptoms. Its strange because I had 2 crises this year with negative test results. Does anyone else have anything similar? My neurologist thinks I still have MusK instead of being double seronegative. I just find it so strange and because of this, it has been very difficult for me to get my medicine approved by insurance