World was created in 1.15.2 and stepped through snapshots:

20w12a

20w14a

20w15a

20w16a

20w17a

20w18a (current)

Rain selectively passes through solid blocks, can be "fixed" by removing and replacing said blocks with any other block. appears to ignore slabs as well underneath single blocks in affected areas. 

I first noticed this in 20w16a but saw bug report MC-179845 and left it alone since it is reported fixed in 20W18a. Appears not to be. The attached screenshots are most noticeable, but it's raining in canyons at Y < 32 in occasional odd places