Welcome to Communities and (maybe) vSAN.
From what you have said above, my first guess is that you have ended up with flat.vmdk instead of vSAN Objects here which will not work for a number of reasons and should be remediated.
This can be validated by checking whether you have flat.vmdk files in the VMs namespace folder.
This can be remediated by cloning the VM to a new VM, selecting the vsanDatastore as destination, applying a valid Storage Policy and using that instead of the current VM (and if for whatever reason that doesn't work or isn't possible then just use old-school vmkfstools -i and with -W vsan flag).
If this is not the case then please provide more details.
Yep they came across as flat.vmdk files. I have done as you suggested and they seem to be working fine. Thank you very much!
More than happy to help and glad I guessed right on first try (if it walks like a duck and quacks then it is probably a flat.vmdk ).
Just a little bit of further information for anyone that stumbles across this post in future:
Other issues may be encountered from using flat.vmdk on vsanDatastore including the namespace running out of space once 255GB has been written to the namespace in total (including the ~1-2GB used without having flat.vmdk in there), failure to extend disks due to insufficient space, failure to take/grow snapshots etc.