I'm seeing this as well. Running 2.1.7658 on Linux (Ubuntu 18.04).
I've noticed that the lastUsed field for the profiles in launcher_profiles.json
doesn't look to be being updated. It's set to "1970-01-01T00:00:00.000Z" for all profiles, which is probably why the launcher doesn't know which one to show (it defaults to sorting by last used date).
As a workaround, I manually updated that timestamp for the latest-release profile, and the launcher now selects it by default.
I suspect this is the same issue as https://bugs.mojang.com/projects/MCL/issues/MCL-12075