Took me a couple reads to put this one together. Based on that you mention being able to see they're associated with someone else's account, you mean that you got a unit that was not factory reset before being given to you right? (And you can't do it yourself because they're Apple-locked for theft reasons.) Not that it got stolen and now you can't find it?

Then you say you went to the store, were given a replacement, and it turned out the same. You didn't try out in the store whether these ones are functional? Which makes sense because why wouldn't they be, but could have saved the trouble. You haven't tried going back to that store, perhaps even at the same time and day of week so that you are likely to get the employee who remembers that they provided support in this situation (by handing out a replacement unit, in this case)? Then you could immediately try out, before taking it out of the store, whether this unit is functional

Either way, they have an obligation to meet the claims put out in marketing statements, including features like FindMy since they say on the website it can do that. If you've exhausted the easy options, you can always take it to something like small claims and get at least a refund that way. Often, a letter saying you intend to suffices