If I turn on the maximum bitdepth for a sequence every clip in this sequence gets a lot brighter (like 3 stops). If you play the clips without high quality playback enabled they get back to their normal brightness. It would be fine if you had to enable the high quality playback to view the clips correctly but if you turn on the comparison view in the program monitor, it shows the clips with their normal (too dark) brightness again. This is really annoying as you cannot see how the footage looks correctly graded while playing the clip or if you want to look at them side by side.
The clips in my sequence are all 12bit.
This was not the case with the 2018 version.
Sorry for this frustrating occurrence. This is the first I've heard of such a case. If you have a moment, would you mind copy/pasting the contents of this bug report to user voice? Premiere Pro: Hot (3081 ideas) – Adobe video & audio apps
That way, the devs involved might be able to take a closer look, because that's where they spend most of their time. I'll also point some of my colleagues to this post, as well.
In the meantime, you may want to return back to the previous version for color critical work until we get this issue sorted out.