Existing publication in Google Scholar profile vanished
There is nothing much to do except for things which you already did and can try again:


Add/remove the publication manually
Submit a request for correction
Contact Google Scholar directly for help about this specific problem (https://support.google.com/scholar/contact/general)


The Scholar system is highly automated, and their automatic methods for parsing/counting of citations can change abruptly and cause general fluctuations in the level of citation. I have noticed it is quite common to see my citation count suddenly increase or decrease by as much as 100 citations (and in one case it changed by more than 500).

Alternatively you can use other citation count resources such as Web of Science or Scopus.