Any Other approach I can do in Appending with Choice List (Fixed) multipicklist

Summary:

may need other approach in appending for a multipicklist

Content (required):

I'm trying to append it on a multipicklist that has an existing data however whenever I save it the selection gets cleared out I have checked all of the scripts that are in the system however there is none that should clear it out the 'XPR' should always be there whenever the StageId is on 111 or 222 and should not overwrite the existing one.

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):

if(StageId in [111, 222] && Awarded_c != null){