I use K-9 Mail on my Android phone (Motorola) and an old RCA Android tablet. Love it.
Except a few weeks ago only on my tablet the email address autocomplete when I was composing emails stopped working. I have to type in the entire email address and then K-9 recognizes it – before I could type in a few characters and would be prompted with a list of matching email addresses to choose from (very handy!).
It still works that way on my Android phone – just not my tablet – and I am pretty sure that K-9 has been updated on both devices.
I did not find a setting to switch that behavior off/on.
I have the exakt same problem. Using K9 9.0 on Android 14, both up to date. K9 is not suggesting / completing any recipient addresses at all. This holds true for contacts on my phone (i.e. stored via the Google Account on the phone). I like to point out that I am well aware of the missing functionalty of auto-completion for other mail addresses (e.g. ones you sent an email to before). But now it is not working in a way it did for years. Kindly have a look into this, devs.
Thanks!
Sounds like your phone rejected the Contacts permission for K-9. Some phones do that quite aggressively when not using the app for a few days. You can turn this off on the app info screen of the Android settings app. There you can also grant the permission again.