We are aware of this bug that for some players the skill changes don't save or that they revert after restarting. We will resolve the issue as soon as possible.
We have a workaround for you. Upgrading your skills to rank 5 requires 270 item power of your character. However, some piece or pieces of your gear are not at item power 270, which causes this bug. Check your gear and, if possible, replace those pieces of gear with item power 270 pieces and then upgrade your skills. Alternatively, you can infuse these items to meet 270 item power.