Seller photographed the installed adapter to prove my spare arrived

Two listed: one fitted, one spare. Same part reference. I've highlighted both

Then ask them to account for the second item specifically. They may be answering the wrong question because the message subject says missing adapter and the person replying sees one on the robot.

Subject now says spare adapter missing, installed adapter received. Running out of ways to say two.

Was the spare meant to be loose in the robot crate, or in an accessory carton? That could direct the warehouse search, though it does not change the quantity.

Good question. Ask for their packing record for that specific item, rather than another general package photo. If they say it was sent separately, there should be a shipment they can identify.

Did they find a packing entry, or are they still discussing the installed part? It would be useful to know whether the clearer wording got this to their warehouse team.

Warehouse admits the spare wasn't packed. They want to include it with our next order

We have no next order planned. I've asked for a separate shipment and checked the other bags too.

At least the quantity dispute is over. Get a definite answer about separate dispatch before this becomes a promise attached to an order you'll never place. Admitting the omission and sending the part are still two jobs.