Description

How to reproduce:

• have at least 2 sites set up
• create a channel section with the propagation method set to “all” enabled for both sites;
• with an entry type that contains a matrix field
• the matrix field is in the cards view mode with the propagation method set to “none”
• go to the non-primary site, create an entry in that channel and try to add a new nested element;
• notice “Attempting to save an element in an unsupported site.” error

Reverted the change from #16136 to fix the reported issue.

Added a different fix for #16136.

Related issues

#16158
#16144
#16176
#16196