This is textbook test fatigue. What you are describing is not regression — it is overload.
When you do 3-4 full mocks per week plus daily vocab review plus daily listening, your brain does not have enough recovery time to consolidate. You are depositing faster than you are storing.
My recommendation: take one full day off from Japanese. No mocks, no Anki, no audio. Then come back and test again. Most people see their score jump back up.
In the final week before the exam, reduce mock frequency to 1 per week maximum. Focus on reviewing errors you have already made, not adding new load. Your brain needs to settle, not sprint.