This is an issue with the aggressive memory management of the LTEvo/One X -- apps are much more likely to get killed and it looks like the monitoring of SystemPanel App is falling victim to that.
1) You can set SystemPanel App to "high priority", which requires that an icon show up on the taskbar. However, the memory management is so aggressive here that even a high priority thread can get killed. I can personally demonstrate this where I would go into Chrome browser, start opening links as new tabs, and then eventually the SystemPanel App icon will just disappear from the taskbar.... outrageous.
2) Strangely, my personal solution was, in addition to setting SystemPanel App to high priority, to get rid of using Sense altogether. I switched to ApexLauncher for my homescreen, which I found to consume a lot less memory, and inexplicably, SystemPanel app has not been killed as of late.
Let me know what you guys find.