Messing around with the undocumented GET accounts/:account_identifier/registrar/domains which is faster and a whole lot less chatty than the documented POST accounts/:account_identifier/registrar/domains request…
I’ve found it behaves inconsistently than other paginated APIs. When one passes ?page=1 and per_page is greater than result_info.total_count, the result[] is empty! 
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This does not occur with other APIs such as GET accounts?page=1.