Wonder what time of day you do these intervals.
For me to do a lengthy VO2 interval in the morning is hellish, though I see guys doing them fasted at 6am!
For me I feel like my lungs are ready late afternoon to early evening. Something I might fail in the morning is smashable merely hours later.
What @splash says might be true though, it can simply be a case of your VO2 being relatively close to your FTP and you’re doing a harder interval that the plan assumes.