The Spectre Illuminator seems to leave behind shadows from smooth lighting on certain entity blocks.
Known entity blocks include Cable Facades (from Applied Energistics 2) and Framed Slant (from Blockcraftery) - see pictures.

Both entity blocks can be retextured with other block's textures, so this might be what's causing the issue.

Version 1.12.2-4.2.7.4
Project Ozone 3 A New Way Forward modpack