Like Kido said, it shows endure icon. Its not a bug, may be 3-4 years back the icon was different as i remember it not showing the endure icon long before. So, if you really want to change then you can try.
In your GRF
1) Search for data\luafiles514\lua files\stateicon\efstids.lua
Line No. 107
EFST_LKCONCENTRATION = 105,
2) Search for data\luafiles514\lua files\stateicon\stateiconinfo.lua
Add this at the bottom
StateIconList[EFST_IDs.EFST_LKCONCENTRATION] = {
haveTimeLimit = 1, posTimeLimitStr = 2, descript = {
{"Concentration", COLOR_TITLE_BUFF},
{"%s", COLOR_TIME},
{"Increases ATK & HIT"},
{"Decreases DEF"}
}
}
3) Search for data\luafiles514\lua files\stateicon\stateiconimginfo.lua
Add this under Priority Red - " [PRIORITY_RED] = { "
[EFST_IDs.EFST_LKCONCENTRATION] = "LKCONCENTRATION.TGA",
4) Add your desired .tga file at the effect folder of your GRF and make sure you name it lkconcentration.tga - data\texture\effect\lkconcentration.tga
If you don't want it to display icon then you can try to remove lkconcentration.tga (will work only if you have done all the above steps except step 4)