Bug: (Dor 5.1Pro / macOS Sonoma 14.6.1)
Dorico is not accepting “-7” as a valid input for min7. I enter “D-7” and “nothing happens” - the dialog just disappears. (I tried “C-9” and it registered as C♭9 
?)
Entering "CM7" Dorico interprets as C7 (I would expect Cmaj7). Even more confusing, if there is a ♯/♭ in the chord root-note, the “BbM7” is interpreted to be B♭mi7.
There’s no bug here. The popover syntax is NOT what will appear in the score. It is merely the command you use to tell Dorico what quality you want, which will then appear in the score according to how you’ve told Dorico you want it to look with your Engraving Options / Chord Symbol settings. Using a + or - in the popover is for altering intervals, so - does not mean minor in the popover syntax. Your C-9 syntax tells Dorico to input a C chord with a flat 9 suffix. You can have minor appear as a - in your score by setting the appropriate Engraving Options setting, but you don’t type that into the popover.
It’s consistent with the way popovers work in Dorico. When I type 3:2 in the tuplet popover, it doesn’t mean I literally want to see 3:2 if I’ve set my Engraving Options to display “Number only.” Dorico creates a 3:2 tuplet and displays it based on what I’ve specified in my settings. Same thing with chord symbols. This isn’t like Type Into Score in Finale.
Go through and set up all of your Engraving Options / Chord Symbols so your chords will automatically appear as you’d like in the score. When inputting use the syntax specified in the manual for those qualities and Dorico will automatically format your symbols to appear the way you’ve specified. You do need to use the correct popover syntax for these though:
Thank you @FredGUnn for the follow-up!
#2 is a Bug: The manual page you linked to has a note
“M and m entries for major and minor are case-sensitive.”
While I would personally expect “CM7” = Cmaj7 and “BbM7”=Bbmaj7, the fact is that currently “CM7” = C7 and “BbM7” = Bbmin7 so there is a logic breakdown somewhere.
Re #1: I guess I am wrong. Apologies. Clearly I thought X-7 = Xmin7 would be standard behavior but the devs disagree…
Using “-” exclusively for extension-alterations seems strange since there are few cases I can come up with that would require -9 instead of a b9. (As an aside- Seeing C♭9 on a chart would mean “B9 dominant” to me, not “Cadd♭9”, but whadoino🤷) The manual only has two examples of using a dash: One is a “-5” as “b5” and the other, add-9. I would think those are exceptions for the common use case of X-7 to be accepted as Xmin7, seeing as X^ is accepted as Xmaj7. So, I guess I could only make the suggestion that - followed by 7 be accepted as “mi7”.
The appearance of that one that will depend on your Engraving Options of course. I have my EO settings set so suffix modifications appear in parenthesis and superscripted. That way I get this by default when I type in C-9 with my settings:
I can’t think of a situation where I’d actually want that, although I’ve wanted C(b2) before, but at least the parentheses and superscripting make it clear the flat isn’t modifying the root.
