I suddenly started getting the exact issues you mentioned here while playing through PO3, with Optifine and Seus installed, but with the added annoyance of a sunny day suddenly looking like an overcast one, when looking in different directions. I found that it was the Botania mana pool causing the issue, and removing the pool fixed the problem. It does seem odd though, as on a previous playthrough I never got the lighting glitches when using the mana pools. There does seem to be some odd interactions with the pools and either Optifine or Seus shaders.