This is probably due to the fact that the 'current line highlighting' is processed before the 'code highlighting'.
The background color set for a given code highlighter, overrides the generic line highlighting.
In terms of columns; if column (or fix width char) 0 to 15 are occupied by the highlighted code, the gerenic current line highlighting starts from col 16. If the code line is a rather long one and lines are not wrapped, the line highlighting becomes invisible.
Reversing thr order, i.e. placing the line highlighting routine at the end, could remedy this situation.