I had this same problem recently and with some digging found the following in limits.conf
maxresultrows = <integer>
* Configures the maximum number of events are generated by search commands which grow the size of your result set (such as multikv) or that create events. Other search commands are explicitly controlled in specific stanzas below.
* This limit should not exceed 50000. Setting this limit higher than 50000 causes instability.
* Defaults to 50000.
Setting this higher resolved the issue.