Subscriber stops receiving mid-run with QoS2 and 200KB payloads (still connected, no errors)

[ooredm]

Describe the bug

MQTTnet client subscriber stops receiving additional messages mid-run under QoS2 + large payload load while remaining connected. Broker reports all messages acknowledged.

Which component is your bug related to?

• Client

To Reproduce

I’ve added the scripts used to reproduce the issue in this repository: https://github.com/ooredm/mqtt-bug

• A Python publisher
• An MQTTnet-based listener
• Alternative listener implementations using HiveMQ and paho-mqtt

In my test run, I published 10,000 messages with a payload size of 200,000 bytes each.

I’m aware that the payload size is relatively large. However, it reliably reproduces the issue, and I would still expect this scenario to work correctly.

Using this setup as an example:

• The MQTTnet listener receives 7,229 messages
• The paho-mqtt listener receives 10,000 messages
• The HiveMQ listener receives 10,000 messages

Expected behavior

Subscriber continues receiving until it reaches the published message count (10,000).

Additional context / logging

MQTTnet version used: 5.1.0.1559
.NET SDK: 10.0.102
OS: Windows 10.0.22631 (win-x64)
Broker: Mosquitto 2.1.2, default config

Code example

https://github.com/ooredm/mqtt-bug