Instant VM Recovery means that you started the VM from the backup repository in order to be up and running quickly. But in that case, it is not the definitive restore as it is running on the backup storage and most probably not on the production storage. That's why you need to finalize the restore (in the background) and that is also why there is an avhd(x) running at that moment.
It is explained in detail here: https://helpcenter.veeam.com/docs/backu ... tml?ver=95
If you think you need more information from this documentation, please let us know so we can have a look at it