What are the civic rights for other cultures?
The proportion of slaves seems about right, only POPs with civic rights citizen or noble will promote, other cultures cannot.
Integrating other cultures will also help with their happiness, that ultimately is producing the unrest that reduces the province loyalty. Alternatively, you can give some cultural decisions to improve their happiness without integrating them.
The governor is not loyal, try to bribe him, give him free hands and get it out. Replace it with a high stats governor with 0 corruption and if possible of the same culture than the majority of POPs of that region.
To check what is producing the lost of loyalty in the province, hoover your mouse over the loyalty bar and post the picture.