I am reporting a similar bug to the issue rasied in March 2019 https://community.powerbi.com/t5/Issues/Slicer-switches-from-Dropdown-to-List-when-using-Bookmarks-l...
The difference is I am not using bookmarks. I am using Desktop Version: 2.74.5619.621 64-bit (October 2019).
Other solutions to this issue such as switching off the visual header or editing the slicer in Power BI Service have not resolved the bug.
When I edit the slicer in Power BI Service and change it from List to Dropdown it looks fine until I save the Report and switch back to reading mode. When I make the switch to reading mode the dropdown automatically reverts to a list.
Please take a look at the screen recording of this issue.
I raised this in the community and was advised to raise an issue https://community.powerbi.com/t5/Service/Dropdown-slicer-not-displaying-correctly-in-Power-BI-Servic...
Some screenshots
Desktop/editing mode
Service/reading mode
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