Throughout the process of working on images, I frequently must switch Treatment modes back and forth from Black and White to Color. In Lightroom Classic CC 7.5, the channel mixing panel no longer preserves the opposite treatment's settings when switching between treatments.
The issue can be replicated by changing the Treatment to Black & White, altering the mix settings in the B&W panel, then toggling the treatment to Color and back again. The black and white mix settings will have been reset. The same holds for the opposite — working in Color, then changing to black and white and back again.
This completely foils workflows in which one must toggle back and forth between modes in order to refine mix settings (e.g. assessing the image in color, switching to b&w to make changes, and so on). As a photographer who must often produce both color and b&w images, I have traditionally kept both versions in the same file in order to consolidate my library a bit. This problem prevents me from doing so, meaning I would need to go back and reproduce years-old edits on tens of thousands of files if that data has been lost.
I am not familiar with how version 7.5 handles the metadata of edit histories or whether it differs from prior versions of Lightroom, but I expect that many photographers' livelihoods (and sentiments) would be saved if this problem were reversible without losing the information that previous versions of Lightroom Classic kept intact.