VM Ports blocked from local network but not from Proxmox host

[special_case]

I've been having an issue where various VMs become spontaneously unreachable from local machines, but are still reachable from proxmox itself. I don't have any firewall turned on in proxmox or the VMs, but it's acting as if there is one. I can't make heads or tails of this -- it just starts on its own and, in at least one case, fixed itself on its own after ~24 hours.

Here is an example. On the VM, ports 22 and 8123 are being listened on.
From an external local machine:

> nmap 192.168.0.8 -p 22,23,25,8123,80
Starting Nmap 7.93 ( https://nmap.org ) at 2022-09-08 13:39 PDT
Nmap scan report for 192.168.0.8
Host is up (0.0056s latency).

PORT     STATE    SERVICE
22/tcp   filtered ssh
23/tcp   filtered telnet
25/tcp   filtered smtp
80/tcp   closed   http
8123/tcp filtered polipo

Nmap done: 1 IP address (1 host up) scanned in 1.25 seconds

From Proxmox:

> nmap 192.168.0.8 -p 22,23,25,8123,80
Starting Nmap 7.80 ( https://nmap.org ) at 2022-09-08 13:42 PDT
Nmap scan report for 192.168.0.8
Host is up (0.00075s latency).

PORT     STATE  SERVICE
22/tcp   open   ssh
23/tcp   closed telnet
25/tcp   closed smtp
80/tcp   closed http
8123/tcp open   polipo

Nmap done: 1 IP address (1 host up) scanned in 0.15 seconds

This has happened with multiple VMs, though they are all DietPi machines.

Details:
Host
Proxmox VE 7.2-7
Running Debian 11 (bullseye)

Clients
DietPi v8.8.1
Running Debian 11 (bullseye)

Host and clients are all on the same ethernet interface on a ProtectLi box