I don’t know why that would be the case, but I wonder if you may be onto something.
I don’t have an explanation for this behavior (neither Playback button that refuses to recognize and play back from the microSD card nor the transient “Failed to load…” message), but I can say that the camera with the “Failed to load…” message that actually does play back from microSD was set up sometime last year, and the camera that gives the “Unable to View Playback” message was set up much more recently.
Strong work, @StevenA!