Add the new values with the Add Values button, not the Add ALL Values button. The add ALL values button will override the existing symbology and re-add all unique values whereas the Add Values button allows you to preserve the existing symbology and add new unique values not present in the Symbol list.
Also, confirm that after importing symbology from layer file, the count of symbols is greater than 0. After import, you have to physically click the header to get the count. If the count beside each symbol is 0 then the values in the source layer file do not match any values in your destination layer.
Based on our discussion, here is the process i recommend:
Import layer file and hit the count header to ensure matching values exist
Click Add Values, Click the Complete list to ensure all possible values have already been added to symbology. Type in new value in the text box and click Add to List
3.Once new value appears in the list, select it and click OK in the dialog
New additional value is now added to the existing symbol list without altering any existing values
Edit new value symbology as needed: