QGIS N:M relations - possibility to edit multiple features?

I have a working QGIS project with multiple relations and all is well until I try the Toggle multi edit mode for N:M relations. I’m working with QGIS 3.10+ (3.14.16) and Windows and PostGIS.

Is it even possible to edit multiple features with N:M relation and, if it is, what buttons should I push to activate this?

When in multi edit mode the N:M relations are hidden and not shown. I thought I managed to do this once, but don’t know what buttons I pushed and now I start to think I was imagining things.

If you need better example I can draft one in English, but I have been following the documentation here: https://docs.qgis.org/3.10/en/docs/user_manual/working_with_vector/attribute_table.html#editing-multiple-fields

I can also contribute to the documentation on this if needed.