Last week, Microsoft has released Windows 10 Insider Build 18312 for Fast Ring Insiders. Some may have already downloaded and updated to it. If not, you can check out Windows Update to download the preview build 18312.1007 (KB4487181).
It has been said that this will be a gradual rollout, so if you do not have it yet, it will be available eventually.
Microsoft’s Chief Insider Dona Sakar even tweeted yesterday that the team was currently engaged in fixing other bugs, including some related to windows sandbox. Some Windows Insiders have been having issues using the lightweight VM solution, and hopefully in the future, Windows Sandbox will be more stable.

Here are a few more things they have fixed:
- Fixed issue resulting in File Explorer unexpectedly having a lock on USBs when trying to safely eject them.
- fixed issue resulting in frequent bugchecks (GSODs) in the last two flights, citing an error with bindflt.sys.
- Fixed issue where a password change can result in the next unlock hanging for AD users.