They've nailed it down to the following logic FUIActionBinding::TryCreate which doesn't check for the InputAction specified.
UDN user was attempting to use Enhanced Input for Common UI but ran into an issue when registering a second UI binding.
In NativeConstruct they're binding an additional action. This widget is a UCommonActivableWidget derivative with bIsBackHandler enabled (with an enhanced input action backing it).
Doing this will produce an error message in the log stating something like the following:
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-193679 in the post.