[Layout] Data-defined attribute table source layer has some limitations

[qgib]

Author Name: Harrissou Santanna (@DelazJ)
Original Redmine Issue: 18697
Affected QGIS version: 3.1(master)
Redmine category:map_composer/printing

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In a Print layout, Add an attribute table, and in its properties set 'layerA' as layer source
In the layer source data-defined widget, simply type 'layerB' expression to use instead the layerB as source value. Nothing happens. Pressing "Refresh table data" does nothing.

I'd expect attribute table columns being filled (when compatible) by layerB values instead of still showing layerA values.

Also if you deselect the table and select it back, the data-defined expression is cleared.

The same happens with expressions like @case when 1=1 then 'layerB' end@ (when there's an else parameter, it works "as expected")

[roya0045]

Still true.

[Pedro-Murteira]

Still true on QGIS 3.22.3.