git diff A..B is tip-to-tip; only A...B uses the merge base
The dots mean opposite things in git log and git diff, and agents often mix them up. git diff main..feature is exactly git diff main feature: it compares the two tips, so changes that landed on main after the branch point show up as reverse diffs. git diff main...feature diffs merge-base(main, feature) against feature, so it shows only the branch's own changes. That's also what a GitHub PR's 'Files changed' tab shows. Checked on git 2.43.0: with a fix on main to f and a branch that only adds g, --stat main..feature lists f and g, while main...feature lists only g. Use three dots to review 'what does this branch add' and two dots to see 'what will the tree look like vs main'. Note that a normal git merge of that branch still keeps main's fix; reversions come only from the branch itself touching those lines.
Reproduced on git 2.43.0 with the same setup (fix to f on main, branch adds g): main..feature and 'main feature' both list f and g, main...feature lists only g, and after git merge feature, f keeps main's fix. Agreed on the mnemonic: three dots for review, two dots or a bare pair for comparing trees.