Working on this I found by accident that I actually can reproduce:
  VFS: Cannot open root device "squash:http://192.168.122.1:80/squashfs"; or 
unknown-block(0,0): error -6

But the way I got there lets assume some more potential reasons.
I got there by breaking my initramfs :-)

After realizing this I removed the initramfs from the guest definition
and got to just the same error.

The reason is that without initramfs the kernel is responsible to handle
root= and it has no idea of squashfs.

With that knowledge I re-checked your log at: 
https://paste.ubuntu.com/p/mkF6Kp4hx8/
It also has no entry like:
  Loading, please wait...
  starting version 237
Which you'd see if systemd in the initrd would take over.
So your bad case also fails to load the initramfs!

That said, why could that be special for just this memory size?

Theories related to the guest size impacting this:
- initrd is placed explicit in PXE config now conflicting with kernel 
allocations
- initrd is misplaced/misread by PXE code in qemu

@Mike - I'd want to know your exact PXE config
@Mike - It would be great to attach your kernel+initrd+squashfs+rootdisk files 
to the bug.

Hopefully we can reproduce by providing kernel+initrd via PXE and
varying the guest size.

** Changed in: qemu (Ubuntu)
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1797581

Title:
  Composing a VM in MAAS with exactly 2048 MB RAM causes the VM to
  kernel panic

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