The bug
Rain and snow cannot be seen through any transparent blocks, such as stained glass, portals, or slime blocks.
Linked issues
is duplicated by
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Affects 19w40a
affectw 20w18a

Confirmed fixed in 20w22a.
Not fixed. affects 1.16-pre2
Can confirm; the bug is still present in 1.16 Pre-release 2.
1.16 pre-release 6
1.16 pre release 7. @@unknown
1.16 pre release 8
happens in 1.16 RELEASE CANDIDATE 1

Your Graphics is set to Fancy. It needs to be set to Fabulous! for this to work.
Now I understand. I have watched video what's the difference and says its fixed.
Can confirm. All transparent blocks are affected.