deleteFromNode recursed into a child subtree and marked the parent dirty but never dropped the child when the delete emptied it. MST nodes are content-addressed, so the emptied node serialized as a real block and changed every ancestor CID — the tree no longer equalled the tree that never contained the key, which is the equality commit-proof inversion depends on. The TS reference prunes here (deleteRecurse: remove the entry when the returned subtree has no entries); we only trimmed at the root. Effect: verifyCommitDiff returned PrevDataMismatch for valid second commits from a real PDS. Any repo with two records whose keys differ in height hits it — the lower key lives alone in a subtree, and inverting its creation must prune that subtree. Found downstream in zlay through the atmoq relay-conformance corpus, where the sync11/commit2-valid control was being dropped. Existing coverage missed it three ways: the lazy-vs-eager delete test asserts the two paths agree rather than that either is right; the interop commit-proof fixtures only delete high keys at or above the root layer; and the verifyCommitDiff test builds its own fixture, inverts an update rather than a create, and ships the whole tree instead of a partial proof. Both new tests fail without the prune: one pure-MST, and one golden fixture generated by @atproto/repo carrying a real partial proof. See REPORT-mst-inversion-prevdata.md.