The issue is that, when installing a beta build for the first time, the "Also include development builds" checkbox in: Options | Updates is not ticked by default.
But, you still get a notification to update to newer beta versions.
e.g. I clean installed 3.08b beta. On launch the "Also include development builds (Beta)" checkbox in Options|Updates was not ticked:
but I did get a notification about the newer beta version 3.10a :