I would recommend the best way to do this is to use the anaplan versions and leverage the switchover functionality.
Then use the switchover functionality within the version settings to take the actuals up until the forecast starts in the forecast version. Switch over is a setting in the version settings or in the module blue print view.
The switch over is explained here:
Select the checkbox if you want the switchover date defined for model versions to be applied to a line item. A line item must have versions applied for the checkbox to be enabled. The list of available time periods depends on other settings in your model; seeTime Period Selection.
You can set the switchover date for versions inModel Settings>Versions.
Switchover does not apply to theActualversion.
However if in your architecture you have not used the versions then you could also do the method you have described.
I believe that your 'Time filter.Actual Months' may have different dimensionality to the module you are doing the calculation in that is why it is not referencing and collecting the boolean condition to display the actual values. If you perform a drill down and it does not show the boolean condition or says not available then this will be the issue that needs to be resolved.