Why does (Exclude) appear in Looker drop-down lists?

I have set up a series of dashboards in Looker Studio that were working fine when using filters with drop-down lists. But now the drop-down lists show the legend (Exclude) and their behavior has changed.
I have tried setting up the drop-down lists from scratch, and the legend (Exclude) is still there.
Has anyone else experienced something similar? Can you help me with this? Thank you very much!