On reviewing the SR, we see that it has been resolved with the below explanation.
Filtering of tree grid to include results on child nodes are not supported. Providing this is not considered in the product design since the primary intention of have a tree grid must be to provide data at top level entry. The tree grid provides an easy way of drilling down the data.
For this use case, it was recommended that the filtering in child nodes needs to be addressed by managing the data that is the source for the tree grid.