The Blueprint Diff tool does not highlight properties that have been changed from a previous version of the Blueprint if those properties are contained in a custom nested category.
No. This behavior also occurred in 4.18.3.
None of the values that were changed are highlighted as having been changed from the previous version of the Blueprint.
The changed values are highlighted as having been changed since the previous version of the Blueprint.
Do not use custom nested categories.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-60388 in the post.