Mail.app's rules are sending the wrong messages to AppleScript

I have the following AppleScript triggered by a Mail.app rule:

using terms from application "Mail"
    on perform mail action with messages theMessages for rule theRule
        repeat with msg in theMessages
            set theText to subject of msg & return & content of msg & date sent of msg
            display dialog (theText)
        end repeat
    end perform mail action with messages
end using terms from

If I select a message, right click and choose 'Apply Rules' it works properly. However, if the script is triggered by an incoming message, it seems to have a random message in theMessages.

Here is the rule:

How do I get the right message?

I'm using High Sierra with Mail 11.2.

Apparently, handling incoming messages with rules is an asynchronous process. When on perform mail action is called, the message is not yet completely updated. Only partial data is available immediately.

A possible workaround would be to add a delay 1 into the script. This give Mail a second to finish updating the message. Here is what the script then looks like:

using terms from application "Mail"
    on perform mail action with messages theMessages for rule theRule
        repeat with msg in theMessages
            -- delay a bit of time for msg to load
            delay 1

            set theText to subject of msg & return & content of msg & date sent of msg

            — do other processing

        end repeat
    end perform mail action with messages
end using terms from