I am surprised you have has had a drop but agree with previous comment that you need to ensure you are comparing like for like first of all ie same test protocol in the same software on the same trainer etc etc.
I can’t comment fully yet based on own experiences but may be able to soon, as I have just finished my recovery week in SSB 1 and my ramp test is on Tuesday ahead of starting SSB HV 2. I seem to be at a plateau for the last 6 months but would not expect a decrease after the last 4 weeks of SSB HV 1.
Differences for me have been that the plan builder gave me 5 weeks not 6 weeks in SSB 1 and I also swapped 1 of the weeks out for lots of Z2 while I was off work over Xmas. This resulted in a higher TSS week than the plan but at lower IF. If I remember I will report back after Tuesdays test.