Based on a comment on CurseForge, quartz slabs are reported as stone slabs. I imagine this issue applies to many vanilla slabs, although I haven't verified anything yet.
Affected Minecraft versions: Likely all, 1.8.9 reported.

(Edit) I can confirm this happens, but only with double slabs. Not sure about the cause at the moment.

(Edit 2) Changing getItemDropped to getPickBlock fixes the issue. This also fixes the upper half of doors, but not tall two-high plants for some reason. I guess WIT just likes sunflowers.