Daniel had responded to this thread 4 years ago, basically saying this isn’t the intended use for this setting and the fact that it works at all is actually the bug!
We really need a reliable way to have better rehearsal letter positioning. As I showed with several examples in that thread, aligning it more or less with the first key sig accidental is very common historically among respected publishers when it begins a system. It would be great if Dorico could reliably support this too. Above the number creates unnecessary vertical space, in the margin looks sloppy (IMO), and at the start of the music makes it a little less convenient to locate as it’s that much further from the start of the system. That’s why aligning with the first key sig accidental is such a common positioning option that Dorico should support.