There have been bugs relating to IDL not passing through transparency properly for a while. I showed a long time ago that a large box-room, with one wall being 100% transparent, receives insufficient light compared to the same wall being made invisible.

That may produce a severe artifact when that is the majority of the light being fed to SSS. There's a lot to go wrong in that chain.

Post a bug report with SM. If you don't it may be treated as minor, since so few people write to them about it.

---

Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)