Affected forms of commands:
/data get entity @s
/execute as @A run data get @s
/execute if data entity @s
The first two using the other target selectors of players (@A, @p or @r) or a specified type argument can get suggestions, while the last cannot.
Minecraft 1.20, Fabric loader 0.14.21, NBTac 1.1.2.

I'll try to fix it in the next version, sometime next month I think.

Partially fixed, now /execute if data should work properly. As for issue with @s, I know I promised to fix it in a month, but it's not simple to fix, yet rather a small issue. I'll try to fix it sometime in the future.