Alt does not follow BAlt in QHover

Evening,
I have a good BAlt trace and the IMU AccZ, AccX, AccY vibes are within suggested limits, but Alt just stays constant. How do I persuade Alt to use the BAlt values?
I am flying a twin prop tailsitter in QHover using a Matek H743 Wing V3 flight controller. I have no GPS unit.


Link to the log below.
https://drive.google.com/file/d/1-w4Y72BFdtrwYwol0B7jQov6Km6DvdZB/view?usp=sharing