The option "Show sort header in icon modes" in File Display Options is unchecked. It works as expected in all folders except Network and Quick Access, which still show the sortable column headers when in Thumnails mode.
Not a bug, or at least not a bug in Opus.
Those folders are delegated to the Windows shell to display, since Opus can't add much value by handling them natively. It's up to the shell how it displays them. I'm not sure if there is a way to configure whether it shows the sort headers in icon/thumbnail modes or if it always does it with no way to turn it off.
That's interesting. But what's a little bit odd is that Windows (10) Explorer doesn't show the column headers for icon views, for any folder. Even odder is that for detail views it does display the column headers, but the headers don't have sort buttons. Instead they provide grouping options. I guess it means either there is a way to configure it or else Explore itself doesn't use the shell for display. Any thoughts about what's going on?
Edit: Explorer's column headers provide filtering options, not grouping like I said above.
Explorer configures the shell components a little differently to what anything else which uses them gets. I suspect it's for compatibility in general, or to differentiate it. A lot of these changes are private to Explorer, and probably not documented as an API so that Microsoft can keep changing things in every OS update without having to keep supporting each iteration's old look & feel at the same time.
Microsoft no longer aim to provide a general platform on which to build apps that are all consistent with each other and the OS itself. The just make things for themselves and their own software, and everyone else gets the scraps or has to make their own, and nothing stays consistent even between different first-party apps. 