Reaper’s mark is bugged. It’s easily timed on training dummy.
If you take just Sigil of Mortality (mark of death) with no points in Reaper’s Mark, the Mark of Death lasts for 2 seconds. The base duration is working correctly.
If you put two points into Reaper’s Mark (supposed to be +100% duration), the new Mark of Death lasts for 3 seconds. (should be 4 seconds)
If you put four points into Reaper’s Mark (supposed to be +200% duration), the new Mark of Death lasts for 4 seconds. (should be 6 seconds)
It looks like there is a numerical error somewhere in your code causing it to be +25% per point instead of +50%.