I do not fully understand, but it is normal that a filter on one clip has unique parameter values for the same filter on a different clip. If you want a single filter for a collection of clips, you need to put those clips on a separate timeline video track and add filters to the track by selecting the head of the track. You cannot attach a filter to a group of clips. However, there is in the timeline clip menu: Apply Copied Filters. This kind of works like a filter on a group except that it simply automates updating multiple filters.