I've never had a dog that stained due to a food I've fed.
Teething causes staining
Allergies (usually inhalent) cause runny eyes which leads to staining
Poor facial conformation can cause the eyes not to drain properly and leads to staining
Eye problems like distichiasis and entropion (both common to Maltese) cause excessive tearing and leads to staining
If for any reason you have excessive tearing you can get a secondary bacterial or yeast infection which compounds the problem
Teething causes staining
Allergies (usually inhalent) cause runny eyes which leads to staining
Poor facial conformation can cause the eyes not to drain properly and leads to staining
Eye problems like distichiasis and entropion (both common to Maltese) cause excessive tearing and leads to staining
If for any reason you have excessive tearing you can get a secondary bacterial or yeast infection which compounds the problem