You would need to update your annotation files to ensure the index values for all annotations match the classes.
Or, alternatively, merge the coco128 dataset with your custom dataset on Roboflow, and export that way. This will solve the issue of the indexes not showing up when validating your model. However, Roboflow makes the index names alphabetical, so they won’t be in the exact same order as coco128, but they will at least show the results for all classes in the dataset.
I found the solution by merging the datasets which was the best for me, but I encountered and error that was affecting the annotations. This altered some of their names converting this to numbers, I’m a bit apprehensive to continue the merging of the other classes until I can find a solution. I have a small but significant sample size suitable for testing but would prove tedious to redo the annotations.
Thank you for your response @Mohamed and acknowledging my digital presence! indeed your efforts were helpful.