If you dive into the NMRA standards and rp’s for DCC you’ll see that 14 and 28 speed steps use the same packet format except that since you don;t need as many bits to indicate 14 posisble speeds, they used one to also indicate the F0 status. The LOW bit (I don;t even want to know what they were thinking). So, for ever odd number, the bit is on, for every even number, the bit is on. 0 off, 1, on, 2 off, 3, on, etc. So if CV29 is programmed for 14 speeds steps and you send 28, the loco speeds up and slows down just fine, but every time you’re on an odd speed step, the lights will be on, and on an even speed step, they will be off. That’s why there’s a setting in CV29 for it - bascially it’s tellign the decoder to use that extra bit to double the speed steps or to use it for the headlight status.
–Randy