To Reproduce:

1. Connect to perforce.

2. Add a material (or any other assets).

3. Submit the Asset.

4. Add a label to the asset before checking out, notice the error:
" .mat.meta is locked and cannot be saved ".
The label is added fine, but Unity still seems to try to edit the file even if it knows that it's locked.

5. The warning is only thrown once, it doesn't appear again when adding additional labels to the same asset.