While it is impossible to get the exact heuristics used by eM Client, I understand from discussing this with them that subject is not the only criteria. Headers like in-reply-to, message-id, reference etc are also considered.
The only experience I have is adding a header to a message to include it in a conversation where it should be, but was not grouped. Not sure (without seeing the original messages) how to remove them from a conversation when they do not have the in-reply-to or other relevant headers. Maybe change the message-id and see if that helps.
To do this you need to save the message, edit it, then import it back into eM Client. If it is excluded from the conversation, then remember to delete the original in eM Client.