As mentioned in the comments below, the bug shown here was fixed in a previous release (apparently U1) of the vSphere client. The trick seems to be the bug only existed in the client and unlike previous versions the client does not prompt to be updated along with vSphere and ESXi. I have tested the GA ESXi (build 469512) and vCenter (build 455964) with a U1b client without seeing the bug.
[Original, unedited post]
This bug has been known for a while and while U1 contained some mentions about fixing it – we now have U1b and the problem still exists.
How to Lose your HA/DRS Cluster in one easy step:
Make sure you have a cluster with HA and/or DRS turned on. Feel free to set any option or enter advanced settings – note that they will all be returned to the defaults.
Note that you can now set HA/DRS up with no further issues – assuming you don’t disable/enable VM Storage Profiles which will wipe you out again.
Note also that existing FT VMs and Resource Pools will continue to function – but you’ll want to fix HA/DRS if you want to manage those features.