Thanks for reporting this, I’ve forwarded this observation to the launcher team. Whether the beta version type pulling the dev UI is a bug or a feature I leave to them.
I recently updated the development UI to 2001, and then to 2004 yesterday. You say the version updated to 2004 without you doing anything, was the launcher running non-stop between versions 2001 and 2004?
We have been working on a rewrite of the launcher UI and just started testing it on the development branch. 
To have a stable release of the launcher UI, please use the release version of the launcher (UI version should be 1058)
EDIT: I just talked to the launcher team and while it wasn’t technically a bug, we will be changing the behavior of the launcher slightly, so that we can deliver more stable versions to beta participants. Later today (once an update has been released) you may restart the launcher in beta to get the old UI back 