This is happening intermittently and I'm not sure of the cause. The keychain still seem to be working properly because the database can be decrypted, but the account password is no longer present.
It seems to happen randomly after a number of launches, but I've only seen it rarely so it has been impossible to reproduce.
One theory is that the OTRAccount object is being duplicated somewhere and re-saved, and that the account password is keyed to the object's uniqueId (UUID), and that the password is saved for the old UUID and not the new one.
Another theory is that when the password can't be fetched from the keychain (from the background?) the account object gets re-saved at some point and removes the password.
Tracking in Zom: zom/Zom-iOS#191
Attempt to fix missing password bug #594
Hope this is fixed!