When using a keyboard media key to start KMail, it starts minimized. Upon first launch, it starts normally and pops up the main window. After the app is minimized to the tray area, pressing the media key opens the app to the taskbar, minimized, and the task switcher doesn't show a window for it. You need to then manually click on the taskbar to open the KMail window. Previous versions (probably two neon releases ago) would always open the main app window upon resuming from the tray area. Note that Krusader is another app I use that is started with a media key, and it works fine. So this seems to be a regression in KMail.
# cat ~/.xbindkeysrc "kmail" XF86Mail works fine. KMail is not minimized when pressing the key
This seems to be fixed in the latest batch of KDE Neon releases over the past week or so. I will leave it open; you can decide whether to close it or not.