If a discovery rule contains the SNMP check which retrieves multiline text value only the first line stores to DB. If the OID will be polled by the item no truncation applied. In the 3.2 the SNMP checks return whole value too.
It seems that the issue is in the zbx_snmp_set_result(), which in the 3.2 expects 4 arguments including the "value type", but in the 3.4 there are only 2 args and no "value type" passed. Inside the function the type set to 'ITEM_VALUE_TYPE_TEXT', while in the discover_service() after the get_value_agent() call goes the GET_STR_RESUTL() call which returns the first line for the text value.