Data defined size legend symbols do not show if you use a field or an expression to scale the symboŀ, and not the DD assistant

[qgib]

Author Name: Harrissou Santanna (@DelazJ)
Original Redmine Issue: 19097
Affected QGIS version: 3.1(master)
Redmine category:map_legend

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If you scale a point symbol size with the data-defined override feature, and you'd later want to display the scales in the legend or layer tree, you'll be able to get a sized symbol legend only if you used the data-defined assistant. Using an existing field or an expression (even when you copy the one returned by the assistant dialog) does nothing: the legend ignores the scaling and seems to be triggered by only the assistant dialog.
This makes the feature less interesting and a bit shaky.

[Pedro-Murteira]

Still valid on QGIS 3.22.4 and 3.24.0.

[alexbruy]

Valid with the latest QGIS 3.33 (2ae70eb)