The skyweave block doesn't seem to work with shaders but works as intended without in the All the mods 9 modpack.
When using the shaders the blocks around the placed skyweave block get darker/lighter depending on the time of the day but it doesn't show the sky as intended.

Custom shaders and modded shaders can't really run side by side, and I we don't intend of supporting a custom subset of shaders.