The person who leaves a group conversation may be reducing noise rather than rejecting company.

Leaving a thread is usually read as a statement about the people in it. Much more often it is about volume: a phone that will not stop, a hundred messages about a plan that concerns two of them, a need to hear one thing at a time. The exit is administrative rather than personal. It rarely means the friendship has changed.

If someone goes quiet or steps out, message them directly instead of interpreting it. Keep them in the loop for the parts that involve them. And if you are the one leaving, say something brief and warm on the way out so nobody has to guess.