TCP is different to UDP. With TCP you cannot send “messages”. You send a stream of bytes. If the length of a payload does not correspond to the length of the send stream, the receiver can detect it, but cannot recover the connection (i.e. the message alignment). Thus, SNMP4J will close a connection, if it detects a message encoding error (i.e. incorrect lengths).
Why the NET-SNMP utility does not reopen the TCP connection is unknown to me. It also could be a completely different reason, i.e. that the NET-SNMP tool does not increase its boot-counter.
You can better find out the real reason by activating DEBUG log on both ends.