Is zed/xayah/camille ult supposed to eat things like vi/naut ult?

yes. all those ults have a untargetable time frame in it. so if the ult hits during the untargetable time frame the ult does nothing

Most ults go on cooldown once used, even if they don't end up hitting their target due to untargetability. So yes, intended.

They didn't 'do nothing' exactly though, as they did force out the corresponding enemy ults (or zhonyas).

The ult goes on CD for like 5 seconds if your target goes B before it hits, or if your target dies before it hits them.