Its probably because there is a max resolution limit that is already set on your Premiere Pro. Try increasing the Max Res allowed in your Premiere Pro project settings. In order to do this, need to do the following:
1.) Go to “File” > “Project Settings in Premiere Pro.
2.) Under the “General” tab, look for the “Max resolution” option. By default, this should be set to “Auto.”
3.) Set Max Res higher than the resolution of your adjustment layer. For example, if your current resolution is 1920×1080, you can try setting the maximum resolution to 4K (3840×2160) or even higher if possible.
4.) Click “OK” to save your changes.
This change should allow you to scale up your adjustment layer without the image turning black at a certain point. Best of luck there! 🙂