The PRs may be orthogonal but still be dependent. Feature X depends on improvement Y which also needs bugfix Z. You might go and implement X in a branch, tweaking the codebase as you go, but split the branch apart for review. You put X/Y/Z up, but X contains Y and Z, which means you can't request reviews for X without Y and Z merging, or else have a bunch of extra code that gets in the way.
Let's say that Z has an error (some assumption that does not hold), and needed to be reverted. How does that impact X's viability? I wouldn't trust any reviews of X after that.
I strongly believe that PR should be compared to the main branch, and not rely on unmerged code. Unless you merge everything together in one go. And in the latter case, everything should be reviewed together.
I think the answer is - it depends! This is why we make good money. I don’t think there’s a hard and fast rule here to apply.