  Unlike other academic bibliographic databases, Google Scholar intentionally
operates in a way that does not maintain coverage stability: documents that
stop being available to Google Scholar's crawlers are removed from the system.
This can also affect Google Scholar's citation graph (citation counts can
decrease). Furthermore, because Google Scholar is not transparent about its
coverage, the only way to directly observe coverage loss is through regular
monitorization of Google Scholar data. Because of this, few studies have
empirically documented this phenomenon. This study analyses a large decrease in
coverage of documents in the field of Astronomy and Astrophysics that took
place in 2019 and its subsequent recovery, using longitudinal data from
previous analyses and a new dataset extracted in 2020. Documents from most of
the larger publishers in the field disappeared from Google Scholar despite
continuing to be available on the Web, which suggests an error on Google
Scholar's side. Disappeared documents did not reappear until the following
index-wide update, many months after the problem was discovered. The slowness
with which Google Scholar is currently able to resolve indexing errors is a
clear limitation of the platform both for literature search and bibliometric
use cases.
