OK, that gives me more info. I have very few ATLS assets on my system, and none as payware, but using the example in my post above, a payware rolling stock wagon, both assets do appear in the Installed filter list in S20 and both can be selected and added to a layout but the obsolete version has its name greyed out.
I suspect that this is deliberate as the obsolete asset certainly is "installed" (as requested by the filter) and this gives you the option to select it instead of the later current version, if you so desire. Holding the mouse pointer on the asset name in the filtered list for a few seconds will reveal its details (kuid, description, author) which will further inform you as to which version is the current one or which one you want to use.
My theories anyway.