Just tried again. Camera settings are 4k (3840) 24fps manual mode, bird takes off. As soon as I go into Active Track, camera jumps to AUTO 4K (4096) 30fps.
Thank you Gambino for confirming. Just checked and same for me, only active track. Not the case in tripod, tap or others.
Video shooters beware if this when using AT! The settings can be changed to preferred once you have entered AT