Do you use multiple data sources here? And the filter displaying 'multiple values' comes from a different data source than other filters?
There is only one data source.
Please note :- The name single value filter is meant to select one value at a time and not to show just one value . even if you select show relevant values for Size .. if a choosen ID have 50 size values but 3 size are relevant to the selected id it will list the 3 relevant size in the size filter(even if Single select).
single select means here you can select one size at a time out of 3 values .