Glad I found this thread as I have been having the exact same issue. I posted this on the inibuilds discord as well. This also has happened on my last 2 flights. In both scenarios I was at flaps 15/15 flying at about 160kts on Localizer and ready to intercept the G/S. When I select flaps 20/20 in order to prepare to select my Approach speed, all of a sudden the nose pitches up violently and I immediately enter a stall as a result of speed plummeting. What is causing this? Because last time I checked I can have flaps 30/40 as fast as 180kts. At the the time I had 160kts and was well out of the red zone. I have a strong feeling this is because I manually selected Final Approach in the MCDU. I am very sure this is a bug despite what the admins on discord are saying. I was not close to stalling and there is no logical reason for the plane to pitch up so violently while bleeding speed at that rate.
