Why is nested array in UE c++ empty?

Question

C++ / C#

There is a structure:

#pragma once
#include "CoreMinimal.h"
#include "FColumnBoard.h"
#include "FRowBoard.generated.h"
USTRUCT(BlueprintType)
struct FRowBoard
{
  GENERATED_BODY()

  UPROPERTY(VisibleAnywhere, BlueprintReadOnly)
  TArray<FColumnBoard> columns;
};

and class:

#pragma once

#include "CoreMinimal.h"
#include "GameFramework/Actor.h"
#include "FRowBoard.h"
#include "Board.generated.h"

UCLASS()
class CHESS_API ABoard : public AActor
{
  GENERATED_BODY()
  
public:	
  // Sets default values for this actor's properties
  ABoard();

protected:
  // Called when the game starts or when spawned
  virtual void BeginPlay() override;
  // Called every frame
  virtual void Tick(float DeltaTime) override;
  UPROPERTY(VisibleAnywhere, BlueprintReadOnly)
    class UStaticMeshComponent* board;
  UPROPERTY(VisibleAnywhere, BlueprintReadOnly)
    TArray<FRowBoard> rows;
  UPROPERTY(EditAnywhere, BlueprintReadWrite)
    UClass* pawnWhite;
  UPROPERTY(EditAnywhere, BlueprintReadWrite)
    UClass* pawnBlack;
  UPROPERTY(EditAnywhere, BlueprintReadWrite)
    UStaticMesh* cube;
  UPROPERTY(EditAnywhere, BlueprintReadWrite)
    UMaterialInterface* color;
};

It has the following constructor:

PrimaryActorTick.bCanEverTick = true;
  this->board = this->CreateDefaultSubobject<UStaticMeshComponent>(TEXT("BoardStaticMesh"));
  this->SetRootComponent(this->board);
  const int32 sizeColumn = 118.0f;
  FVector boundsColumn = FVector(sizeColumn * 0.02f, sizeColumn * 0.02f, 0.01f);
  for (int32 i = 0; i < 8; ++i) {
    FRowBoard row;
    FVector forwardVector = (FVector::ForwardVector * i - FVector::ForwardVector * 4) * sizeColumn * 2 + FVector::ForwardVector * sizeColumn;
    for (int32 j = 0; j < 8; ++j) {
      FColumnBoard column;
      FVector rightOffset = (FVector::RightVector * j - FVector::RightVector * 4) * sizeColumn * 2 + FVector::RightVector * sizeColumn;
      FString name = TEXT("col_");
      name.AppendInt(i + 1);
      name.AppendInt(j + 1);
      auto col = this->CreateDefaultSubobject<UStaticMeshComponent>(FName(name));
      col->SetupAttachment(this->board);
      col->SetRelativeScale3D(boundsColumn);
      col->SetStaticMesh(this->cube);
      col->SetMaterial(0, this->color);
      col->SetRelativeLocation(rightOffset + forwardVector);
      col->SetVisibleFlag(false);
      column.column = col;
      column.location = rightOffset + forwardVector;
      row.columns.Add(column);
    }
    this->rows.Add(row);
  }

Before exiting the constructor, there are 8 structures in rows, each of which has 8 array elements. But in the editor and in BeginPlay rows somewhere loses nested arrays and only eight elements remain, each of which contains zero elements.

How to fix?