First of all, there is no real “Google sandbox”. There is what some people call the “sandbox effect”, which is simply the principle of trusting a webpage that has stood the test of time.
You mention the domain is 11 years old, however, domain age has absolutely nothing to do with SERP ranking. Search engines don’t rank domains, nor websites, they rank individual web documents.
How old is the webpage content that dropped? If it’s something new then naturally it has not stood the test of time and has not yet earned trust.
What else has changed? how many pages were affected? Was it just one keyword on one page, or was it many different keywords and many different pages that dropped in the ranking?