For me, I limit how hard my start is so I can keep power above FTP for the interval. My logic is that I want to stress both my cardio system and legs with an effort that’s above FTP.
If I start too hard - similar to you - my power drops close to or below FTP - mainly due to my cardio system not being able to keep up. Maybe a VO2max physiologist would say thats OK, since the main goal anyway is to stress the cardio system. But I’d also like to train my legs to be able to put out power at VO2max-type efforts, and not just have my cardio system be operating at VO2max.