That's not a gotcha on Proxmox projects but a gotcha everywhere, SWAP on ZFS needs a lot of special care to avoid that it needs to allocate memory during swapping memory out (e.g., if low on memory), causing hard-freezes.

It can be made somewhat[0] stable with using no (or a simple compression like ZLE), avoiding log devices and caches it can work, but it's way simpler and guaranteed stable to just use a separate partition.

[0] even setting the myriad of options there still exist reports about hangs, like https://github.com/openzfs/zfs/issues/7734#issuecomment-4167...