I have a report with NO Bookmarks.
And only for a few users, sometimes the dropdown slicer is showing as a list.
It is happening randomly, not able to reproduce the issue on-demand.
Always try the latest version of Power BI Desktop and check Supported browsers. You may also create a support ticket for assistance if necessary.
I ran into the same issue. Deleting and recreating the slicer in PBI Desktop then re-publishing solved the problem for me.