Your services may run in separate VMs, but there are still some dependencies between them. You need to know, and think about, all the dependencies between your VMs.

For example, they share a common network interface (the one of the host machine). That is a dependency. If one VM is able to clog the network interface (and maybe your crashing one is doing exactly that), then it is clogged for all the other VMs too.

To resolve that dependency, you can either put another network interface card in your host machine and let only the pihole VM use it, or run the pihole on a real physical Pi.

You could also resolve the jellyfin's own problem. But resolving the dependency might give you a more reliable system.

I'd check high I/O wait, specially if your all of the vms are on HDDs.

one of the solution I had for this issue was to have multiple DNS servers. solved it by buying a raspberry pi zero w and running a 2nd small instance of pihole there. I made sure that the piZeroW is plugged on a separate circuit in my home.

If the VM crashing is because of high CPU usage on all cores, high IO delay on the storage, or an out of memory situation on the host, that would cause all of the other VMs to struggle as well.

How many cores do you have configured for jellyfin?

Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:

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I'd start at figuring out why your jellyfin VM is crashing

This happens to me when there is an app keeping a file opened on NFS storage mapping

Why are you running full VMs for something that can be put in a container? Sounds to me (without having any evidence or proof) that you’re running out of memory and you’re swapping and it’s taking forever. That’s what causes the VMs to slow/stop.