On live when last stand is used, your max health increases by 30% and your current health is increased by the same value; then, when last stand expires, your current health value is maintained, unless that total would exceed your normal max health.
For ease of explanation, lets say you have 100k max hp and use last stand on live; if you are at 30k hp and use last stand, your current health will move to 60k hp and your max will be 130k. If nothing else happens, when last stand ends your current health will stay at 60k hp with your normal max of 100k.
On Beta, in the same situation, when last stand expires, you instead maintain your current % of max health instead of absolute health. In the same case as above when last stand is up 60k of 130k is 46.15% health, so when last stand expires you'll be at 46.15k of 100k.