I think this is worth reporting as an integration issue rather than normal Rhino layer behavior. Rhino supports parent/child layer hierarchies, and the parent and child layers are separate entries in the layer structure.
If the Forma plugin is treating the sublayer selection as part of the parent selection state, that would explain why deselecting the child also clears the parent. In a normal hierarchy, I would expect to be able to change the selection state of the child without forcing the parent to change.
As a quick test, I'd try the same layer structure in Rhino 8 without the Forma connection/plugin and compare it with the same file while the Forma integration is active. If the behavior only appears with the Forma plugin, that would give Autodesk a much clearer reproduction case.
It would also help to provide a small .3dm file containing just one parent layer and 2–3 sublayers, plus the exact selection steps that reproduce it. That should make it easier to determine whether the issue is in the Forma synchronization/selection handling rather than Rhino's layer system.
Hopefully Autodesk can reproduce it, because having parent and sublayer selection coupled would make hierarchical layer management unnecessarily difficult.
