I assumed that the API was intentionally filtering out log entries, but that isn't correct. Is there some other reason why entries like these appear on Special:CheckUser but not on the API?

(Logs) . . 23:40 . . <redacted> was created
        IP: <redacted>
(Logs) . . 23:40 . . <redacted> User account <redacted> was created
        IP: <redacted>

The bigger issue is that ApiQueryCheckUser.php is replicating code that already exists in SpecialCheckuser.php in an encapsulated way.

Specifically, SpecialCheckUser::doUserEditDBRequest(), SpecialCheckUser::doIPUsersDBRequest() and SpecialCheckUser::doUserIPsDBRequest() should be used by ApiQueryCheckUser.php, for which I think they should be changed from protected to public.

@Niharika is there any plans, as part of CU 2.0, to rewrite the API code base?

I tested this on a Vagrant dev environment and it worked fine:

cugen = site._generator(
    pywikibot.data.api.ListGenerator,
    type_arg='checkuser',
)
cugen.request['curequest'] = 'edits'
cugen.request['cutarget'] = 'TestAcct'
cugen.request['cureason'] = 'test'
cugen.request['cutoken'] = site.tokens['csrf']
data = {
    'cudata': [x for x in cugen],
}

{'cudata': [[{'timestamp': '2020-05-12T18:59:32Z',
    'ns': 2,
    'title': 'TestAcct',
    'user': 'TestAcct',
    'ip': '10.0.2.2',
    'agent': 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/81.0.4044.138 Safari/537.36',
    'summary': 'User account TestAcct was created'},
   {'timestamp': '2020-05-12T18:59:31Z',
    'ns': 2,
    'title': '',
    'user': 'TestAcct',
    'ip': '10.0.2.2',
    'agent': 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/81.0.4044.138 Safari/537.36',
    'summary': 'was created'}]]}

But attempting the same thing on a sample account on enwiki got nothing:

[[]]

Apologies for wasting anyone's time on this. I didn't realize that the API defaults to only 14 days worth of data, instead of defaulting to 90 days like the special page.

In T252559#6130411, @Huji wrote:

The bigger issue is that ApiQueryCheckUser.php is replicating code that already exists in SpecialCheckuser.php in an encapsulated way.

Specifically, SpecialCheckUser::doUserEditDBRequest(), SpecialCheckUser::doIPUsersDBRequest() and SpecialCheckUser::doUserIPsDBRequest() should be used by ApiQueryCheckUser.php, for which I think they should be changed from protected to public.

@Niharika is there any plans, as part of CU 2.0, to rewrite the API code base?

@Huji, not at the moment but we are looking into it - T252051: [Investigation] CheckUser API usage and dependency on special page