Google confirmed a bug that dropped pages from the search index around the weekend of April 5th. Moz data suggested drops on April 5th and 7th, with about 4% of stable URLs falling off of page one. Most sites recovered soon after.
Google dropped pages inadvertently from its search index due starting on April 4. Three days later, Google said it had fixed the issue, only to say a day later that it was not fully resolved.
Read more: https://searchengineland.com/google-de-indexing-issue-now-fully-resolved-315225