Within the Blueprint Editor, it is possible to create subcategories for variables to be sorted into.
Doing so separates variables not nested into subcategories out into standalone, top-level structures.
I can reproduce this issue as far back as the live 4.20.3 Binary (CL 4377374); it is therefore not marked as a regression.
In the Details Panel, the following sections and variables are present
Variables created with the same root category are combined, example:
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-81810 in the post.