Hi all. I have now been working with VMware support team for the last 3 month, and for the last week I have been testing a Bugfix-patch that eliminated the problems.
In a couple of weeks they will come with 6.5 U1 where the fix is included. Here's a short statement from VMware:
VMware Engineering have Root Caused this issue as to how the unmap (Block Space Reclamation) was working on 6.5.
The fix for this has been completed and will be included in the next ESXi release 6.5 U1. This release is currently on schedule to be available next month.