Why is iMessage automatically adding a person back into a new group chat?

I have a group chat in iMessage with four people, let's call them A, B, C, and D.

Today, I wanted to send a message to A, B, and C - but NOT D.

So, I created a new iMessage, added A, B, and C, and crafted my message to them. I sent the message and, on my phone, it appeared to go through successfully and I had a new group chat with those three people.

On their side, however, they received that message from me in our EXISTING group chat with A, B, C, and D!

So, my iPhone sent that message to the existing chat, including D, by itself!!!

I went even further to test this out by completely deleting both the new group chat and the existing group chat. Then, I started a new group chat with just A, B, and C... but when I sent the message, it once again went to the existing group chat including D on the recipients side.

How is it automatically changing who receives my messages?! We have confirmed that only one phone number is stored for each contact - no email addresses. We all have iPhones. We all use iMessage in the Cloud.

[Re-Titled by Moderator]