I’m sorry you’ve lost all emails from your Inbox. That sucks.
K-9 Mail doesn’t have functionality to automatically delete emails. Deleting an email has to be initiated by the user. And by default that will move emails to the Trash folder instead of outright deleting it from the server.
Now, of course it’s possible there’s a bug and K-9 Mail could have deleted your emails automatically. But I believe that’s rather unlikely. If you have downloaded the app from Google Play and you didn’t opt into the beta program you’ve received K-9 Mail 5.600. That version is (sadly) almost 2 years old and is or was used on (without exaggeration) over a million devices. In all that time a problem like yours hasn’t been reported and then tracked down to K-9 Mail having been the cause.
I hope your email provider can restore the emails and provide some information on what happened. Please keep us updated.
Best of luck