We have recently seen volatility in the Event Data API due in part to some queries that generate very large result sets. To improve reliability we will be reducing the limit of rows in a single query from 10000 down to 1000. This brings Event Data into line with the Crossref REST API.
Note that multiple pages of results can be retrieved using cursors, as described in the Event Data user guide.
If you would like to track our solution to this issue, you can head over to Gitlab.