You should only need to sign max. once a month if you haven't used the app for very long time. Otherwise you should not need to sign in often.
We earlier had an issue with iPhones when the phone was running out space, the phone deleted some stored login informations in order to save space - this should be fixed now in latest app updates. In case you do have this still, free some space from the phone so the phone is not complaining about running out space, but, like said, this should not occur anymore. If it does, send email to firstname.lastname@example.org and we can take a look deeper.
Jarkko / Cozify