Swapped out the boards today, and I'm still baffled. While there's no damage visible to the boards themselves or the wiring, it's definitely the amber LEDs that have issues. The whites still illuminate at 100%, but the amber LEDs are noticeably dimmer. One is at about 50%, the other two maybe 25%, and one doesn't illuminate at all.
Rather than continue to rack my brain on "what if's", I'm going to leave well enough alone and move on not knowing what really happened. Despite that, it still concerns me that it happened not long after I started using them, and makes me a little on edge with the new ones.
The new ones are working 100% right now. I'm crossing my fingers it's a coincidence that two boards experienced the identical failure at almost the same time.