However, for me this approach is causing issues with widget extensions which I have yet to figure out.
DejiApps
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The problem is that app groups for ios has to be in the format "group.com.domain.appName" while app groups for mac has to be in the format "TeamID.com.domain.appName".
The problem you and I are running into is that multi-platform approach forces you to use the ios format. You are seeing this warning because you are using the "group.com.domain.appName".
When I split my app up into multi targets and used the proper mac app group format, the "access data from other apps" warning went away.