See repro steps.
Unset and set bIsFocusable on widget's class will allow bIsFocusable to be set and persist on the nested instance.
1. Load pre-4.11 widgets with one widget nested inside another.
2. Nested widget's class has bIsFocusable set to true (the old default pre-4.11)
3. Nested widget instance has bIsFocusable set to false.
4. If bIsFocusable is set to true on the instance, it will revert to false on loading.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-28869 in the post.