whether there is a multisite-wide default new user role in the back-end SQL tables

I do not know multi-site wide default role value for the new users. Every site has its own default role value, like

select * from wp_options where option_name='default_role';

To get this value from site 10 you need to execute:

select * from wp_10_options where option_name='default_role';

This value URE allows to modify. It does not set any multi-site network wide value, which override the value, which admin set when added new user. Value set by admin has a priority.
Did you try to deactivate all plugins and add new user with other then subscriber role then?