In Messenger Lock options, if you choose to signing out automatically when locking messenger and wants to send a message to open chat windows, that message cannot be sent because you aren't signed in when Plus! forces to send the message. Therefore, maybe disabling auto message option when auto-sign out is selected in "when locking messenger", Plus! users won't be disappointed due to unsent messages to their contacts.
EDIT: Please read my different point of view about Messenger Lock from this
post
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