Termination of a deleted pending message acks all messages below it under AckPolicy All

Observed behavior

Setup:

• stream N (subjects n.>, file storage, no limits)
• durable pull consumer C (AckPolicy All, FilterSubject n.>, MaxAckPending 4, AckWait 5m)

1. Publish one message to the stream, read it and accept it to move ack_floor >0 (otherwise when ack_floor is 0: consumer.purge() cleans pendings by itself)
2. Publish 4 messages to the stream: 2 3 4 5. messages 2 and 3 should have one distinct subject
3. Fetch the messages but do not ack. Received messages: 2 3 4 5
4. Purge messages 3 and 4 (by their common subject)
5. NAK whole batch
6. Fetch the messages again. Received: 2 5. And two advisories appeared $JS.EVENT.ADVISORY.CONSUMER.MSG_TERMINATED.MSG_TERMINATED.<stream>.<consumer> for messages 3 and 4, reason "Unacknowledged message was deleted" appeared. ack_floor.stream_seq moved from 1 to 4
7. NAK whole batch again Problem --> 8. Fetch the messages again. Received messages: 5

The message 2 was never acked before and is lost. If any error or NAK happens on step 6-7 client loses the message.

Expected behavior

The same flow as in Observed behavior, but: 7. NAK whole batch again 8. Fetch the messages again. Received messages: 1 4

The termination of a deleted message must not affect other unacknowledged messages when the consumer's AckPolicy is All.

Server and client version

Server: nats-server v2.12.8 and v2.15.0 (latest release, checked today), standalone, default JetStream settings, file storage

Client: NATS.Net 2.6.7 (.NET 10)

Host environment

Windows 11 x64, single server process

Steps to reproduce

Attached a single-file .NET program with all steps. Requires .NET 10 SDK: dotnet run AckAllFloorJump.cs [nats://host:4222]. The program prints REPRODUCED or NOT REPRODUCED at the end.

sample-output-2.12.8.txt sample-output-2.15.0.txt AckAllFloorJump.cs

Submission acknowledgment

• I am a human being writing in my own words and not an AI agent. I will not use an AI agent to communicate on my behalf in this issue, either directly or via copy-paste.