The camera also lost focus on the jet during the record where it went transonic for the first time ever (around 1:01). It looks like they struggled for a while to recover focus against the featureless sky because I would guess they had their lens set to auto focus, which uses a spot or grid of consensus edge detectors. There's no circular polarizer filter equipped because you can see window glare, and that momentarily caused the AF grid to choose the window as the subject because there wasn't (any?) a good enough stabilizer. For a relatively stationary subject like this, it's more reliable to pull focus manually because the camera can be jostled without confusing an AF algorithm. Even better to periodically use spot auto-focus to acquire the sharpest focal plane, then flip it to manual.