The update provides the following fixes and improvements (Referenced from https://support.microsoft.com/en-us/help/4011025/may-8-2018-update-for-office-online-server-kb4011025)
- Assume that you use Office Online Server to save some changes of pictures or some other objects of a Word document in SharePoint Server 2016. When you open the Word document in Office clients or Office Online Server again, you receive a message that states the Word document is corrupted.
- Assume that you deploy two Office Online Servers in a SharePoint Server farm. When you try to coauthor an Excel workbook together with another user by using Excel Online, the workbook is locked and the operation freezes.
You can download this update from http://www.microsoft.com/downloads/details.aspx?familyid=0b6cbe01-ffd6-47fb-8bb6-f457c5e8629b
