Are acked lost and acked unseen the same thing?

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The "acked lost" and "acked unseen" indicate the same event:
An ACK was seen for which the segment it acknowledges is not in the capture file.
You can filter on this using either
tcp.analysis.ack_lost_segment or
_ws.expert.message == "ACKed segment that wasn't captured (common at capture start)"

"previous segment not captured" means a segment was arriving out of order at the capture point.
It could have been lost or is just arriving late at the capture point. A filter on those is
tcp.analysis.lost_segment
Hope this helps,
regards Matthias