The guard that stops a stale list page from reverting a fresh pin was
released on the PATCH's own ack. A list request issued just before the
write is slower than the write, so it lands after the ack still carrying
the old value, with no guard left to fence it: the pin flips back and the
next reconcile pushes that wrong value to the server, making it durable.
Keep the guard until a page actually confirms the value written, with a
cooldown so a row that never returns can't fence itself forever, and drop
it outright when the write fails — the server never changed, so it stays
authoritative.
Also reset the mirror bookkeeping on a gateway switch. mirrored/pending
are per-backend facts; carrying them across a re-home told us the pins
were already pushed to a backend that has never seen them.
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