Thanks for reporting this @Starker3, I can recreate this on demo.matomo.cloud and locally on 4.x-dev. It also seems that the 'Unique Visitors' metric is only available for selection when the current period is Day

This is probably caused by this config.ini.php setting:

; the following settings control whether Unique Visitors `nb_uniq_visitors` and Unique 
; users `nb_users` will be processed for different period types.  year and range periods are 
; disabled by default, to ensure optimal performance for high traffic Matomo instances
; if you set it to 1 and want the Unique Visitors to be re-processed for reports in the past, 
; drop all matomo_archive_* tables it is recommended to always enable Unique Visitors 
; and Unique Users processing for 'day' periods
enable_processing_unique_visitors_day = 1
enable_processing_unique_visitors_week = 0
enable_processing_unique_visitors_month = 0
enable_processing_unique_visitors_year = 0
enable_processing_unique_visitors_range = 0

In which case we should explicitly handle the scenario where the unique visitor metric is already displayed in the dashboard KPI widget and then the current period is changed to a period where unique visitor data is not available. Perhaps a warning could then be shown: "Unique visitor data is not available for year periods. For more info read this [faq]"

Thanks @bx80
In the case of all Cloud accounts by default the Unique Visitor metrics are enabled for both Day and Week.

In this specific customers case the metric is enabled for the Month period as well, which is why I think that there is some data shown in the Sparkline and the Widget shows some data there.

It does look like there is a bug here somewhere with displaying the Unique Visitors metric label. Even with enable_processing_unique_visitors_week = 1 set locally, the label is not shown for periods other than Day.

That one should be related to this piece of code: