The Apple Mail app has some quirks which I recently found unhelpful. I have been getting quotations for some work on the house and when each one came in I moved it to a special mailbox I had created for the project. I needed to contact one of the contractors and went to my special mailbox to find only one email from a different contractor there. I did a search and found the relevant email which said it was located in my special mailbox. After dealing with the reply I went back to the mailbox and on more careful scrutiny I found an arrow with a number beside it on the sole email in the box. I clicked the arrow and there were the emails from all the contractors including the one I searched for. For some reason best known to itself Mail had decided that these particular emails should be linked together.
Lionel Ogden 