Could it be that the baseline of Petaluma is incorrect? (See examples)
No, it’s not that the baseline is incorrect, merely that its metrics are different to those of other fonts. You can adjust the amount of padding on all sides for enclosures to get a more even appearance.
Thanks, Daniel!
Maybe padding settings for a more even appearance can find it’s way into the “font’s recommended engraving settings”…?
Sorry for my meticulousness: 
For my taste the equal-glyph and the digits of a tempo mark are well aligned when using Bravura but too high with Petaluma.
(The difference can be seen in examples attached to my first post of this thread)
I’ve found no way to change this manually, but maybe I am missing something.
This is a small issue, of course.
But what I like about Dorico is that it gives us the possibility to tweak nearly every graphical detail.
For text items you can always use baseline shift, but you can’t do anything to change the relative baseline positions of glyphs in actual tempo items, of course. I think the issue with the tempo mark may actually be that the note is incorrectly aligned in Petaluma Text. I’ll look into that in due course.