setting a StyleBox bg_color with a @tool script makes the colour flicker to black

### Tested versions

4.3dev5

### System information

Windows 10

### Issue description

see title

### Steps to reproduce

something like this....

```swift
@tool

var stylebox = StyleBoxFlat.new()

@export var color: Color:
set(value):
stylebox.bg_color = value
```

### Minimal reproduction project (MRP)

/

Reproduce the issue using the provided @tool script and StyleBoxFlat color setter in Godot 4.3dev5 on Windows 10. Start by confirming when the color flickers to black and use the minimal reproduction information in the issue to trace the editor-side update path. Done means assigning the exported color no longer causes the StyleBox background to flicker black.

Written by the indexing model from the issue text.