It happens when viewing energy data for a single day, on columns between 18:00 and 00:00.
First image is with Untracked consumption hidden, second is with it enabled (notice the discoloration on blue line)
Version is 2025.2.4, HAOS. I don't want to make a GitHub issue immediately as I don't know if it's specific to my setup. I'm happy to make one if it's actually a bug though.