Hello!
I know there are currently a lot of requests for BYG compatibility, but I haven't quite found anything that breaks things regarding that until now. With fully up-to-date versions on 1.16.5 (BetterFoliage-2.7.1-Forge-1.16.5.jar and byg-1.3.4.jar), we are experiencing that the Meadow Grass block becomes textureless and see-through when full-block grass is enabled.

The temporary solution for this is to go into Mod Options (or config file) > Better Foliage > Connected Grass > Off.
This isn't a great option for those of us who like to have connected grass, though.

(found by cms1121)