Signal team needs to address this.
“Signal should warn users who are likely using insecure IME apps”
Apparently Signal added a new support page for this: https://support.signal.org/hc/en-us/articles/360055276112-Incognito-Keyboard
And a new warning is currently in Android beta.
“Signal Is A Good Start, But Beware”
Personal instance of nitrohorse (nitrohorse.com).