If I can figure out how to add screenshots I will.
Ubuntu 18.04.3, GNOME 3.28.2, I7-7700K, 16 GB RAM, 512 GB nVME SSD, GTX 980 (using Nouveau display driver), Minecraft v1.14.4 (java), no mods.
I had a powered rail (with a redstone torch adjacent), another powered rail, a normal rail, another powered rail, a cobblestone block, and a stone button on the top of the cobblestone. This is my standard "end of the line" setup so I can jump in the waiting minecart, hit the button, and go jetting off to wherever that rail line goes. This one happens to be in a place where the rail line continues to grow as I mine further back. As I mine further back, I will tear up all but the first powered rail, lay 36 normal rails and then build the termination I described above.
This time (never happened before), when I tore up the termination (except for the first powered rail with the redstone torch adjacent), the stone block under where the second powered rail was, is darker than the other stone blocks around it and when I lay a rail onto it, it turns the rail black. As I said, I have never seen this before and I've done this hundreds of times.
I've uploaded a couple images so you can see what I'm talking about.