This used to happen occasionally, that double-clicking an item in a dock wouldn't register the d/click. The item [doesn't matter what type] registers as selected, but doesn't action. Rick-click & select anything from the drop menu works, including Open.
I'm sure at one point that quitting & re-launching DragThing would at least temporarily re-enable the double-click, but today that doesn't seem to be working either.
I'm going to try a reboot, with & without restoring previous apps & see if one of my regular apps might be causing this... will report back...
It's not reproducible, unfortunately. afaik, the order in the list isn't the order they're launched any more, so that makes my 'test' little more than voodoo. Swapping the order back now makes no difference [which my further research shows it wouldn't]
The only tentative conclusion I can now come to is the mere action of removing it & re-adding made the difference... yet that makes as much sense as shaking the chicken bones at sunset on a Tuesday with an R in the month
I'll mark it down to "who knows" & see if it ever happens again.