I have had exactly the same issue in the past.
I went with 20 min FTP test as:
1,. that’s how i tested before
- the ramp test result i knew was way too low
The results helped me recognise that i (like many others on here) are a little vo2 constrained.
I down adjusted my vo2 and supra-threshold workouts slightly to match this during SSB and also SPB - i found that by the end i was nudging up all sweet spot/threshold and over-unders but still down adjusting my vo2 sessions.
TBH from what i have felt ( and read on here) as long as you are in the 105%+ on the vo2 sessions its no issue.
My advice would be to follow the higher number and adjust along the way. The numbers and %'s of FTP can’t be perfect for each rider so its good to use a bit of intuition.
I found following what i ‘thought’ was my ftp worked the best and i tried to adjust each session to achieve the desired training effect, but this does come with practice i guess (not that i pretend to have got this mastered at all!)
hope this helps.