That was the current version when I reported the problem in June - Plexamp Release Notes - #60 by SimonXCIV
TBH not surprised the logs don’t show anything as clicking the link launches Safari not Plexamp, so I am guessing Plexamp wouldn’t even be invoked by iOS?
Seems to me whatever mechanism Plexamp uses to associate itself with shared links is not working in this instance.
edit: they are on latest 4.8.2 now and same issue, opens Safari initially not Plexamp