Does the paho client have a buffering mechanism when lot's of message arrived with high concurrency in subscription ?

We use a broker (Mosquitto 1.4.11) and use mqtt v3。Our java application introduced paho(version 1.2.5) and subscribed to a topic using callback（MqttCallback.messageArrived()） to receive mqtt v3 messages. When another program published a large number of messages to this topic with high concurrency, then istopped sending. We found that the paho client in our application seemed to be blocked, and the application did not receive all the messages through subscription just part of messages. When any message was published to the topic again, our application could receive the messages that were not received before. Does the paho client have a buffering mechanism? Can this function be turned off? It feels like a bug.

Start by reviewing the Java Paho 1.2.5 subscription path around MqttCallback.messageArrived() and compare its behavior with Mosquitto 1.4.11 under concurrent publishing. Reproduce the reported case with a high message rate, determine whether messages are buffered or delivery is blocked, and document whether the behavior is expected or a bug.

Written by the indexing model from the issue text.