I’m experiencing exactly the same behavior.
The only explanation that comes to my mind is that the model has seen that I can hold higher power outside than inside. Therefore, it expects me to exceed the targets on outdoor workouts.
If that really is the case, I would prefer that a harder workout is chosen when switching a workout to outdoor. Because now, I can only guess by how much I should exceed the targets outdoor to get the intended training stimulus and with it a stable prediction.
I think you may be right, my estimation is the gap between inside/outside for me is 20w. (mostly due to the different power meter locations)
I did try to exceed to workout target outside (workout IF:0.92, achieved 0.95) but it still gave me a prediction reduction.
Final update as I’m giving up with the latest AI version…..
Saturday outside easy ride, (did with no power just HR) and prediction dropped from 297-288.
Swapped Sunday plan easy ride for a VO2max and prediction went to 302 (
)
Completed Sunday V02max as accurately as possible (outisde) and prediction dropped to 297
It seems regardless of how I complete (exceed target, on target or just with HR and no power) outside rides drop the prediciton and with no comment as to why, it’s very demotivating. (Yes I’ve still done the work and fitness is improving but……
)
The next release is supposed to fix it. Hopefully it does for you.
I am seeing exactly the same. I hit power records on my hard sessions outside and the predicted ftp drops. Even worse on an easy ride. It dropped another 10 watts overnight after an outdoor endurance ride with power records for over 1 hr period. I am very relaxed that I am getting fitter, but I honestly don’t understand how Jonathan got AI ftp increases as he rides outside with this version of the software.
As others have said, this is a bug that’s our top priority at the moment.
It affects Outside Workouts and FTP Predictions.
Your Detections and workouts should all still be accurate, so try to ignore the noise for the time being. 
Turning off FTP Prediction might help with your sanity while we get this one sorted out.
Sorry for the trouble!! 