Have you checked your traffic and ranks? This could just be an issue with index reporting. If traffic and ranks are stable then no need to worry.
If however traffic has declined, there is certainly an issue, check:
- have you received a penalty
- use site: command to see what URLs are actually showing Google index
- fetch and render to see how Google see's your pages
- run a crawl of your site using Screaming Frog or other such tool
- is there issue's with 404 / 500 or No response pages
- has dev deployed anyting like moving to https without applying 301 redirects
I think first is to determine, if this is an actual issue or not. Then if it is, run some serious analysis to determine cause and apply a fix.
Many thanks,
