I am not a Proxmox expert, so I might be wrong...
However, I wouldn't recommend setting the container as privileged, since you might introduce a vulnerability to your host by doing so.
By default, your Proxmox host should already have working NTP synchronization, and your LXC containers get the time from your host. I believe that if you set the container to privileged, Pi-hole will also attempt to update the time on the host, which is not necessary since the host already does that. So all you accomplish by setting the container as privileged is introducing a potential vulnerability to your entire host/cluster.
So, in my opinion, the best fix for this is to simply empty the ntp.sync.server and set the ntp.sync.interval to 0, as mentioned in the marked solution by DL6ER.