The internal tracking in your plasma tube is perfectly normal. Electricity follows the path of least resistance. With plasma tubes this resistance is an inert gas. The resistance from the rear of the electrode is providing less resistance than the internal portion of the electrode. This is the ionized gas path you see in your tube. It will not cause localized heating and will not damage your tube or reduce its lifespan.
The ionized gas between the end electrodes forms an antennae and transmits the required frequencies. Internal tracking between these electrodes will certainly compromise performance and indicate a problem. Your tube is functioning correctly.