Looking at a list of questions, it might not always be clear to the user that a filter is still applied:
When applying a filter (e .g. adding a tag and clicking apply) the filter mask is shown and it's obvious that this filter is active. But when clicking the "x" (where the filter icon was) the filter mask is not shown anymore but the filter is still active. In this case, there's no visible sign of a filter being applied.
The same can happen for "favorite tags" if those get edited by clicking the filter icon.
It seems that the filter icon is kind of a toggle switch to show/hide the filter mask. If hidden and filter are applied the icon could somehow reflect this.
Looking at a list of questions, it might not always be clear to the user that a filter is still applied: When applying a filter (e .g. adding a tag and clicking apply) the filter mask is shown and it's obvious that this filter is active. But when clicking the "x" (where the filter icon was) the filter mask is not shown anymore but the filter is still active. In this case, there's no visible sign of a filter being applied. The same can happen for "favorite tags" if those get edited by clicking the filter icon.
It seems that the filter icon is kind of a toggle switch to show/hide the filter mask. If hidden and filter are applied the icon could somehow reflect this.