We've made the container of an Entity declaration implicit in XLSForm which makes things simple for simple forms but may make reasoning about more complex structures harder. First we need to fix the regression and then we'll keep thinking about what documentation or improvements we can provide to make this easier to reason about.
Your example @MinimalPotato has this structure:
container1
field1
container2
field2 save to entityList1
container3
field3 save to entityList2
You want to save field1's value to both lists. There's no way to express saving a single field's value to multiple lists so you need to "forward" field1 with calculates to at least one of container2 and container3. I think forwarding to both as you have done is the easiest to understand.
Technically, per our spec, it should be possible to have an Entity declaration at the container1 level to save field1 and field2 to entityList1 and then only forward field1 into container3 and have the entityList2 declaration in container3. That feels really confusing to me so I would not recommend it. The issues discovered in this thread suggest it probably won't work as it should currently.
We'll need to develop and write up some best practices as we all write more forms with it!