Thank you! So I recreated your case and for me it worked (using openLCA 2.7.0):
- demo process

- product system (a library product and process and a result)

- gives this result


Some questions and comments:
Why is your impact category called "core"? I would not really call a flow a "process A" (it is as if you call a process "flow A" or "impact category A" - this is easily confusing).
If an element in a product system "is not taken into account in the calculation" it will not be shown in the contribution tree, in your case you see it is taken into account but has 0 impacts -> this is either a matter of scale (results are < 0.000) or, more likely, the impact category does not fit.