Hello. Roy, I saw you also posted on this thread.
I reproduced the missing IsValid issue you mention in UE 5.1.1 and found that it’s the same bug as that one:
UE-149464 “For Each” Node doesn’t show up in UMG Graph until an Actor BP is loaded
For example, if I start a blank project and create a widget blueprint the IsValid node (just like the ForEachLoop node) don’t appear in the blueprint context menu. If I open an actor blueprint at any point, both those nodes start appearing. So the repro conditions I believe are: don’t open an actor blueprint yet and then start editing a widget blueprint.
I see you’ve found workarounds. 
Opening an actor BP would also be a workaround. This won’t be fixed anymore for UE 5.1 but is fixed for 5.2. You can consider cherrypicking the fix if you build the engine from source. Log into Github and check out this fix.