I observed this on several Samsung Tablets with Android 10/11:
After installing K9Mail I placed several account Widgets for different accounts on the main screen. In the beginning below the icon I saw the name of the account (as I know it from my stone aged S6 with Android 6), but after some days I noticed, that below the widgets no longer the name of the account is shown, but ‘K9 Mail’ instead. So I have 3 identical Icons on my screen, but I can not distinguish for which account they are.
Are you using the “K-9 Accounts” or the “K-9 Unread” widget? Which K-9 Mail version are you using?
I’m using K9Mail 5.600.
The widgets are ‘K-9 Accounts’
“K-9 Accounts” is creating launcher shortcuts. The name is set during creation and it’s the job of the launcher to remember that. If the name is displayed correctly in the beginning, but then switches to “K-9 Mail” later, it’s probably a bug in the launcher.