Weird re-location issues

I have duplicated the chest and all its pieces. I have also applied all modifiers so everything is a separate object. I have one collection that comprises all of the straight pieces - the wood planks and the straight metal bars, the metal lids, and another collection that comprises all of the curved objects (curved metal bars on the lid, hinges, and rivets).

Now, I have deleted all of the materials from this set of duplicates. But when I select everything in my straight collection and join them together, the items in my curved collection get reset (location-wise) to their original counterparts, and I cannot for the life of me figure out why.

Any thoughts?

I can, of course, move them back, but I just wondered what on earth is going on.

[update] I think I found the issue, I had tried to make the items single-user, but no joy. Then I thought maybe I had a parenting setup somewhere (how I do not know). So I cleared Parent (kept transformations) and everything works fine now :-)