Very interesting point. Seems like Qualtrics will need to decouple action planning spaces from dashboards to allow for this kind of situation, and I don't feel that the situation is a rare one, either.
Though not ideal, one idea that seems the least worst is simply using the first project's dashboard to add new action plans to. You lose the ability to create an action plan at the click of a button for subsequent project dashboards' items, but you can create them manually in the first one to keep them all in one place (the downside being that they need to go back to this original dashboard).
Sorry - there doesn't currently appear to be a good solution for this.
The only thing that might correct for this given Qualtrics' current functionality is a process/mindset adjustment in the org. Getting their action plans down to a 3-month focus. Once you're past that window, you focus your action plans not on the past, but on whatever the data in the current dashboard is showing you. I've no idea if that's the right course or way to think about action planning - I just know we did quarterly goal-setting when I was at Qualtrics. Hopefully, people can make and execute on these shorter-term action plans, and if anything needs to be brought forward, then they accept having to manually create it in the new dashboard (¯\\_(ツ)_/¯).

@Matt_B Something has come on-line since my last response. Have you seen Manager Assist? It has a Plan of Action section that pulls all action plans from their different dashboards into a single space.

Sounds like this may solve the concern we discussed previously.