It is my general understanding that the letter underlines, be they in a menu, set of buttons, or similar are what the program exposes to the screen reader. If the program itself isn't set up to pass this information to the screen reader, there's nothing the screen reader can announce (and, no, I won't even consider the very old "screen scraping" method of getting things as I don't know that any modern screen reader employs this mechanism).
But I get the same result with NVDA that you do, in that the S for Save and the D for Discard are shown on those buttons with underlines yet there is no, "Save button, ALT+S" or "Discard changes button, ALT+D" announcements when you're on them, just the button names.
This could be an NVDA issue, or it could be that Thunderbird is doing something peculiar with what it exposes, or doesn't expose, to NVDA. But that would be for the respective developers to work out between themselves. All an end user can know is that this is a problem, but you can't be 100% certain where the root cause lies.
I have no idea how common or uncommon this might be.
Brian - Windows 10 Pro, 64-Bit, Version 2004, Build 19041
A gentleman is one who never hurts anyone's feelings unintentionally.
~ Oscar Wilde