"Replace with child" does not work well with the NamedSlot widget after reloading the widget blueprint in Contents Browser.
Please follow the steps carefully.
1. Create Widget Blueprint and name it "Test1".
2. Open Test1 and add a NamedSlot widget to Canvas Panel.
3. Compile and save the Test1 and close BP editor.
4. Create another Widget Blueprint and name it "Test2".
5. Save Test2.
6. Select Test2 in Contents browser and right-click and perform "Asset Actions > Reload".
7. Open Test2.
8. Add Test1 widget to Canvas Panel of Test2.
9. Add Overlay to NamedSlot of Test1 instance.
10. Add Button to that Overlay.
11. Right-click the Overlay and choose "Replace with... > Replace with child".
Expect: Overlay is replaced with the child, Button.
Result: Overlay remains, and Button is disappeared.
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-112751 in the post.