Neo4j remove existing relationships itself during adding new relationships - neo4j

Question

I have a code responsible for creating new relationship between two nodes.
At first It get a two nodes by property (pk), check if such relationship between those nodes already exists and if It doesn't not, create one.
public void createNeo4jGraphLink(#Nonnull final String childPk, #Nonnull final String parentPk) {
final UUID measureId = measureService.startMeasure(MeasureService.MeasureEvent.NEO_PERSIST);
Node child = neoRepository.findOneByPk(childPk);
Node parent = neoRepository.findOneByPk(parentPk);
checkIfLinkingAllowed(child, parent);
if (child.getPartOf() == null || !child.getPartOf().contains(parent)) {
if (child.getPartOf() == null) {
child.setPartOf(new HashSet<>());
}
child.getPartOf().add(parent);
neoRepository.save(child);
}
measureService.fixMeasure(MeasureService.MeasureEvent.NEO_PERSIST, measureId);
}
I can see that number of relationships rises in db, but one moment it sharply decrease and so on.. What reason can lead to this behavior? I've tried to use #Transactional annotation, experimented with propagation and isolation of transaction and tried without transactions at all, but it all does not work. Btw I use Spring Data Neo4j repositories. Spring Boot 1.4.4.RELEASE.
Child and parent entity' type is "Node" which is defined as a follows
#NodeEntity(label = "node")
public class Node {
private String pk = UUID.randomUUID().toString();
#JsonIgnore
private Long id;
private String name;
private Set<Node> partOf;
private String type;
public Node() {
}
#Property(name = "pk")
public String getPk() {
return pk;
}
public void setPk(String pk) {
this.pk = pk;
}
#GraphId(name = "id")
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
#Property(name = "type")
public String getType() {
return type;
}
public void setType(String type) {
this.type = type;
}
#Property(name = "name")
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
#Relationship(type = "PART_OF", direction = Relationship.OUTGOING)
public Set<Node> getPartOf() {
return partOf;
}
public void setPartOf(Set<Node> partOf) {
this.partOf = partOf;
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Node node = (Node) o;
if (!pk.equals(node.pk)) return false;
return true;
}
#Override
public int hashCode() {
return pk.hashCode();
}
}
I perform adding the relationship 1800 times, but as result have 62 relationships. Code does not throw any errors. While processing those 1800 requests, I can see in database there is any number of relationships (less then 1800), but as result only 62.
Thank you for any information