When querying to find associated accounts for a list of account ids, some ids in the results appear to be missing many of the associated sites. I have only observed this in multi-page responses. For example, if I query:

https://api.stackexchange.com/2.2/users/1144996;6913068;6453670;1751488;73042;1840350/associated?page=2&pagesize=100

in the response, account 1840350 contains these sites and reputations:

    Bioinformatics Stack Exchange                        123
    Coffee Stack Exchange                                248
    History of Science and Mathematics Stack Exchange    101
    Open Source Stack Exchange                           101
    Pets Stack Exchange                                  101
    Worldbuilding Stack Exchange                         101
    elementary OS Stack Exchange                         101

but if I query for only account 1840350, using:

https://api.stackexchange.com/2.2/users/2.2/users/1840350/associated?page=1&pagesize=100

Then I see that 1840350 is active on 35 sites, with 7180 reputation on Biology Stack Exchange and 1513 on Photography.

I presume this is a bug that has to do with the implementation of paging. If I request page 3, the response has an empty items list.