seems it's fixed in the daily installer builds, and thus it should be safe to assume the fix will be part of the next installer (alpha 8?). grab daily build copy here https://d-i.debian.org/daily-images/amd64/daily/ , it works for me.
according to a guy on irc://irc.oftc.net/#debian-boot : <adsb> (...) the next build will automatically pick up the current versions and work as expected
