White balance auto-adjusted half way through recording
A1: Links like these mention that ProRes 'bakes in' the white balance (as opposed to storing it as separate metadata): blackmagicdesign.com forum ; reduser.net forum ; reduser.net forum ; cinematography.com forum ; bmcuser.com archive

(I am surprised this is not more obviously stated on the web anywhere - it seems to be assumed knowledge)

A2: I did end up computing the least-squares solution that brings the 'bad' pixels over to the 'good' ones in RGB space. This gave me a 3x3 transformation matrix that could be applied to each pixel of the bad frames. After this their colour looked very much like that of the good frames.

In MATLAB this is as simple as reshaping one's image matrices into long 3xN lists of pixels (A = the bad frames, B = the good frames) and computing M = B/A, i.e. solving MA=B for M.