This is a curious one. I recently discovered that Windows 10 Mail frequently got into a state where I still got lower case letters when pressing Shift. I discovered it happens when a Keyman keyboard is active, and goes away when I change to a regular Windows keyboard.
Even more curious, it seems it happens only to keys that are not modified by Keyman. I did a new test keyboard that modifies b, B, d and R if I press them after a modifier key. With this keyboard, pressing SHIFT in Mail has no effect except for B and R.
I notice the same issue in other new Windows 10 apps, like Calendar. (Maybe the right terminology for these is Universal Windows Platform (UWP) apps).
My Keyman desktop version is
My Windows 10 version is Windows 10 Home 20H2.
My Keyman keyboard has this code:
begin Unicode > use(main)
group(main) using keys '/' + 'b' > U+0253 '/' + 'B' > U+181 '/' + 'd' > U+0257 '/' + 'R' > U+0158