If you are entering a listing for a property and the tax information (tax amount, assessment, mil rate, etc.) that fills in from the Parcel ID is not correct, you must first save the listing as Active, and then go back into it, make the corrections and save it again in order for those tax fields to retain your changes. If you were to make the corrections to the listing before it has been saved as Active, Matrix's built-in validation feature, that runs when a listing is first saved as Active, will revert any changes you have made made back to what is on the tax record.
One scenario where you will run into this issue is when you are listing multiple parcels as a single package. When you are initially entering the listing, you will not be able to enter the combined assessment or tax amounts until after you have saved the listing as Active. You can go in right after successfully saving the listing as Active and enter the combined figures in the Assessment and Tax Amount fields.