Even though that seems like an old question let me give you my 2 cents since I encountered the same problem.
Let me start by clarifying that this question is not about the maxresultsrows that is set by default to 50k, the op is already paginating through the results and he is getting them in chunks of 50k.
The problem here is another limit of 500k. (notice the extra 0)
max_count = <integer>
* The number of events that can be accessible in any given status bucket
(when status_buckets = 0).
* The last accessible event in a call that takes a base and count.
* Note: This value does not reflect the number of events displayed in the
UI after the search is evaluated or computed.
* Default: 500000
You have to change that limit.
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