I downloaded a self-generated map from garmin.openstreetmap.nl (via the link I received in an e-mail).
The windows installer installed the complete map I requested in Basecamp : no missing tiles.
The gmapsupp.img I copied to my Etrex misses several tiles (at least, they are not rendered on the Etrex).
How to go ahead to have this issue resolved (I already tried it two times, with the same result) ?