FYI: This looks like some bug in core where a custom dimension is not found in https://github.com/matomo-org/matomo/blob/3.6.0-rc1/core/DataTable/Filter/PivotByDimension.php#L322

The ID of the dimension is CustomDimension.CustomDimension1.

The problem is that the factory only includes hard coded dimensions in https://github.com/matomo-org/matomo/blob/3.6.0-rc1/core/Columns/Dimension.php#L830 vs all dynamically included dimensions are ignored in https://github.com/matomo-org/matomo/blob/3.6.0-rc1/core/Columns/Dimension.php#L773

Just a guess... You can reproduce it with Custom Reports and eg using a Visit dimension plus a Custom Dimensions dimension.