Hi Dominic,
I have rebuilt your test application in Ubuntu VMs and can reproduce this issue.
I have so far discovered that it is appears that not all the pages to be written are sent from the client.
It seems that after a long sequence of reads requests from the client the next page which gets sent from the client to be written has skipped one which then leaves an unwritten empty region of 4096
(which will be all zeros) bytes.
So in addition to prove this is a client side issue as I can reproduce this with Workstation on a Windows host as well as our Fusion products.
Thanks.
Steve