Confirmed here, in vertical reading pane mode. In particular, you get an email notification but you can’t find the email in the account inbox because it’s underneath the top banner. Only by clicking on the top message (actually the second message) and pressing the up arrow key can you navigate to the actual first message. There’s only a tiny bar a few pixels high to indicate the selected message is off the top of the screen underneath the banner.
OS and version: Windows 11 version 21H2 build 22000.739
Installation method: MailspringSetup.exe
Mailspring version: 1.10.3-a476c230