Hey,
It looks like this is a regression from 5.4 that was most likely introduced in this commit: https://github.com/EpicGames/UnrealEngine/commit/5d2571bb36ab37410643e12d5a20909cc95d81c3
It seems possible that child widgets (with no other Enhanced Input bindings in their event graphs) do not properly have bAutomaticallyRegisterInputOnConstruction set to true, and so their parent bindings will not work.
As was previously mentioned, a decent workaround for now is to have a dummy Input Action bound in your child widget which is pinned into at least one node. This should get the child widget to compile with bAutomaticallyRegisterInputOnConstruction set to true and will cause its input component bindings to get set up correctly.
There is a bug report for this issue specifically here (it may take a day or two to become publicly viewable): Unreal Engine Issues and Bug Tracker (UE-268605)