Only since the last SO2021 update it is no longer possible to apply a paragraph style by double-click on style name, e.g., from help
Tip 2: You can also change the paragraph style in the sidebar by double-clicking on the desired style.
As far as I can tell all other methods work, eg, click apply, use the Home>Styles drop down, use the right click context menu.
SO 2021 on Windows 7 Pro
bug? applying style via Styles sidebar
Re: bug? applying style via Styles sidebar
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Re: bug? applying style via Styles sidebar
to be clear: the problem is fixed.