and files in the generated list can be selected to open that file
but when using a command like GO "D:\Pictures\+Upscaled (4K)" FOLDERCONTENT="filefilter=*.png",nodirs,hideext
to list images, then clicking on an image does nothing, to get the default associated application to launch you have to select it from the right-click menu.
FWIW, this on its own works fine here (adjusting the path for one I have):
GO "D:\Pictures\images" FOLDERCONTENT="filefilter=*.png",nodirs,hideext
Do you have Preferences / Viewer / Standalone Viewer / Slideshow / Automatic slideshow turned on? That's the only thing I can think of that might change which file is displayed when the viewer opens.
Edit: Could also happen with a script that automatically lister-links new viewers, maybe.