Hi Diego,
The problem is that the legacy blog comment system had neither a user id for authenticated posts nor an email field for unauthenticated comments so there was no data available to migrate for avatars/gravatars. The new comment system has both a userguid and an email required for unautheticated users so new comments even on old blog posts can get an avatar but older existing comments migrated from the old system cannot.
I agree it is a bug to show the author avatar in excerpt mode. Its fixed now in the source code repository so it will be fixed in the next release. There will be a new BlogDisplaySetting in the next release for "ShowAvatarWithExcerpt" in case someone does want to show it with the excerpt they could set that to true.
Best,
Joe