TI was unaware that it'll differ with the same power meter (in my case vectors) if I ride a wheel on, direct drive or outside.
A few more thoughts on this...
FTP is not a fixed thing, like saying you bench pressed 250 lbs. It is more conceptual, because you are using a shorter test to theoretically project what you might do in a one-hour all-in effort, which is also conceptual and subject to conditions.
So, the key is understanding that FTP is just a tool to help guide your training and racing, but it is not an absolute. An indoor ramp test, an indoor 20 minute test, and outdoor test on a flat, and an outdoor test on a climb would all likely produce different FTP numbers.
IMHO, FTP is most valuable is a training guide. Programs like TrainerRoad test FTP and then use that to index all of your workouts. It does not matter if FTP is perfect as long as it adequately drives workout efforts that make you stronger. TrainerRoad does this really well.
On the other hand, if you want to know FTP for bragging rights, then I would do an FTP test on a 8% - 12% mountain climb on a cool morning. For me, this produces an FTP about 20W higher than an indoor trainer ramp test.
My other important use for FTP is guiding me to my race target power. But again, it is just the initial input that I then refine with actual race experience. For example, on an HIM with a cool run, I learned that I can ride a little over 80% FTP as measured on an indoor ramp test. But, if the run is hotter (in the 90s), then I need to ride around 75% FTP from my same indoor ramp test.