Deviating Visit count results of two different API calls

Hi Matomo Team!

We are using Matomo 3.14.1 at different customer sites, and we recently stumbled upon a deviation of result counts of two different API calls, which we could not explain even after hours of research.
The two methods used are Live.getLastVisitsDetails and CustomDimensions.getCustomDimension.

We’re looking at the number of unique visitors of the site’s second instance (dimension16=up) in the year 2021. The first one seems to deliver the correct results, the second one has unexpainably less.
If we consolidate the result of the first call on userids (dimension12), we get 19 unique users, but the second call gives us only 13.

Here are the calls that we use:



The same deviation is also seen in Matomos build-in Expert View widgets. We created one widget “users”, which listst the 13, but the full Visits Log shows the actual 19.

Is this a bug? Or do we somehow overlook an error made in the build-up of the second query? Or are we comparing apples to oranges, and there is a reasonable explanation for this deviation?
I hope someone can help.

Thanks in advance!

PS: We already set the config setting “enable_segments_cache = 0” since a while, as pointed out in a while ago, and the segment filter actually works, as the results show only the visits from dimension16==up, and not the ones from the first instance, in both calls.