I’m no expert, but I’ll give you my € 0.02. In my opinion the Finder has a problem with associating the ‘stack’ extension, as has been said by @Isaiah and others.
That reminded me that others and I had a similar problem some time ago (in 2016) and I found the forum post in which I participated: RW Project Files saving as Folders - ElCapitan You might download and install the RCDefaultAPP (it still works) and have a look at the association of the ‘stack’ extension.
As you can see in the screenshots, it is associated with RW8 on my Mac, but it can easily be changed.
There might be other apps which can give you similar information, but this little PrefPane restored my sanity a few times, so I’m happy it still works.
