Incorrect post count when linked accounts are authors of the same post prior to linking

**Steps to reproduce**

1. Create an editor account (editor-779)
2. Create one post (test-post-779)
3. Create a guest author account (guest-779)
4. Add guest author to post (test-post-779)
5. Check post count for editor and guest author
6. Link guest author account (guest-779) with editor account (editor-779)
7. Check post count for editor and guest author

**Expected behaviour**

✅ Editor: 1 post
✅ Guest: 1 post

**Current behaviour**

❌ Editor: 2 posts
❌ Guest: 2 posts

**Related issue**

https://github.com/Automattic/Co-Authors-Plus/pull/776

Start by reproducing the numbered account, post, author, and linking steps, then trace how post counts are calculated after the accounts are linked. Done means the editor and guest author each still report one post; the related pull request is linked in the issue for context.

Written by the indexing model from the issue text.