Yes - CSS issue.
position sticky is set inside a container - in your case that would be the div tag that is the parent - it is using display:grid.
display:grid will default to align-items:stretch which will ensure that your 2 columns inside the grid will have the same height (The column with the facets and the column with the product list).
So if you change the grid container to have align-items:start instead, the 2 columns will no longer have same height and that will allow for your facets to stick to the top.
Then you also need to add info on where in the viewport you want it to stick using a top position in css - in this case it will be the height of the header with navigation that you set as the threshold by adding top:170px to your sticky column.
Like this:
