Nicolas from Solspace's Support came back to me with an answer :
I could reproduce the issue and reported this as a bug to our developers.
Still, a patch would be welcome.
We're using Solspace Importer 2.2.5 with ExpressionEngine 2.10
Importer is used to cron import members data from a csv. Everything is running smoothly, except one thing :
Importer is updating existing member accounts while the drop down list in the import settings is set to do nothing.
We're filtering members upon username, we use a unique customer key number ( string like 12AB34 ) as username.
I opened a support ticked at solspace but got no answer till then. Any advice would help, the site is e-commerce, and we're live.