Foliage turns pitch black with skylight 'cast shadows' enabled

Hello,

I have turned on ‘cast shadows’ on a stationary skylight so that the interior parts of my level are lit correctly, the issue is that as I imported some megascans ivy, I noticed that after building light some of it turned pitch black. After spending hours trying to pin down the cause, I realised that turning off ‘cast shadows’ on the skylight would make it look much brighter.

I’ve attached some screenshots showing the comparison, as well as my skylight and lightmass settings.

https://i.imgur.com/5eV8KJD.jpg

https://i.imgur.com/fugrmMu.jpeg

https://i.imgur.com/flbWtTG.png

https://i.imgur.com/USMmWGa.png

I’d like to know what I could do to fix the issue while keeping cast shadows enabled. Thanks for any help.

Update:

It seems that the issue happens when the foliage is flipped. Comparison screenshots with the same foliage group flipped by 180°:

https://i.imgur.com/jkx8dho.jpg

https://i.imgur.com/mnXPk0d.jpg