set_custom_data for TileData with negative y-axis in TileMap does not work correctly

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### Tested versions

4.2.1.stable

### System information

Windows 10 - Godot 4.2.1.stable - Vulkan (Forward+)

### Issue description

When `TileData` is obtained from the `get_cell_tile_data` function of `TileMap`, `set_custom_data` does not work well if the Y axis of the obtained `TileData` is negative. After executing `set_custom_data`, `get_custom_data` for the same TileData will contain a different value.

```gdscript
for map_point in get_used_cells(0): # Prepare a TileMap that returns [(0, -1), (1, -1)]
var tile_data = get_cell_tile_data(0, map_point)
tile_data.set_custom_data("coordinate", map_point)

var tile_data2: TileData = get_cell_tile_data(0, Vector2i(0, -1))
var coordinate = tile_data2.get_custom_data("coordinate")
assert(coordinate == Vector2i(0, -1)) # (0, -1) is expected but an error occurs
```
Maybe `get_custom_data` is not working correctly.

### Steps to reproduce

It is reproduced by running the sample project.
https://github.com/inoutch/godot_issue_set_custom_data

### Minimal reproduction project (MRP)

https://github.com/inoutch/godot_issue_set_custom_data

Start with the linked minimal reproduction project and run the negative-y example. Trace TileMap.get_cell_tile_data together with TileData.set_custom_data and get_custom_data, comparing the stored value for the reported cells. Done means the reproduction preserves the expected Vector2i(0, -1) value after set_custom_data.

Written by the indexing model from the issue text.