identities do not always adhear to signature setting in composition default

Expected behavior

1. When changing the signature in composition default, all identities should update to this signature, or a prompt should come up asking if it should overwrite all identities.
2. When adding a new identity, it should by default assume the signature as set in composition default

Actual behavior

1. When changing the signature in composition default, identities retain their current signature.
2. New identities have no signature, even though composition default it set.

Steps to reproduce

1. create account
2. create 1 or more new identities
3. set a composition default signature
4. create more identities
5. realise it's weird and you are glad someone reported this (thank me later)
6. thank me

Environment

K-9 Mail version:
5.400

Android version:
7.0

Account type (IMAP, POP3, WebDAV/Exchange):
IMAP

Start by reproducing the issue in K-9 Mail 5.400 on Android 7.0 with an IMAP account, following the listed identity and composition-default steps. Trace where composition defaults and identity signatures are stored and applied. Done means existing identities update or prompt on signature changes, and new identities inherit the configured default signature.