This is so annoying, and so baffling, that I feel compelled to ask about it here.
I have an iPhone and a mac, my wife has an iPhone and a mac. Her iPhone is set to start all conversations from her phone number. Mine is set to start all conversations from my email address.
If I send a message to her email address, it will appear on her phone in a different chat, than if I send a message to her phone number. Despite there being only a single contact for me in her phone, with both my email address and phone number in it.
Yet on her Mac, both messages will collapse into a single chat. It makes no difference which method is used to contact her, Messages on the Mac handles them as a single chat to a single person - which just makes sense.
The reverse of course is true with her sending me messages to my phone number or email address, and of course on my Mac everything collapses to a single chat as well.
What gives with this? Is this really expected behavior? Is there no way to fix this so that all Messages to a given person, regardless of the method used, will go into one single chat on their device?