Hello Lucas,
"some issues in out vSAN Streched cluster when some VMs disappeared from inventory and from vSAN Datastore."
So my first question would be what exactly do you mean by "disappeared", do you mean unavailable/inaccessible or permanently gone? - the only legitimate times I have seen such things (e.g. excluding something or someone deleting stuff) is when people have data Objects stored as FTT=0 (almost exclusively unknowingly) and lose a Disk/Disk-Group, aside from this it should be clear what happened to the data.
I would strongly advise to open a Support Request with vSAN GSS if this is not well understood already.
"Why would this happen? What are possible causes for that to happen?"
I would start with checking do you have leftover stale CMMDS entries from replaced Witness(es), from Master/Backup node (*should* be the same) this can be checked with:
# cmmds-tool find -t PREFERRED_FAULT_DOMAIN
# cmmds-tool find -t HOSTNAME
Other than that, potentially there was some other issue with CMMDS, I can only really think of one (which is actually a long knock-on effect from issues causing /scratch to be unavailable) as issues in this area are exceedingly rare (which they SHOULD be as this service places a critical role).
Bob
