I was playing on a server while it was raining so I decided to work on something near my roof when I realised rain could fall through, this has been posted before but not with any way to replicate it, it seems to only occur when you're looking at a stair block otherwise it blocks the rain like it should.

Attached is two screenshots both looking at the roof but screenshot two actually has the stair focused (in it's black border).

I know I'm using a texture pack but it wouldn't make this bug occur.