This is a regression. Tested in //UE4/Release-4.27 CL#18319896
Nesting a Structure in another Structure results in a 'Cannot parse value' until the data value is updated. If the second structure is added to a data table; updating the first structure can result in the data entered into the data table being lost.
The icon has a green check mark and the tool tip says 'Good to Go'
There is a red minus symbol and the tool tip says 'Cannot parse value'
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-157882 in the post.