XML editor behaves erratically

version: Boxes 40.3-stable (stock with Fedora 34)

When adding custom data (in my case ACPI parameters in qemu:commandline format) the editor in VM properties > System > Edit XML behaves erratically.

When I paste the XML changes and click Save, they vanish, but if I simply repeat the steps again and save, then it saves normally.

Steps to reproduce:

1. Create VM
2. VM properties > System > Edit XML
3. Paste before final tag close:

  <qemu:commandline xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
    <qemu:arg value='-acpitable'/>
    <qemu:arg value='file=/usr/share/seabios/slic.bin'/>
    <qemu:arg value='-acpitable'/>
    <qemu:arg value='file=/usr/share/seabios/msdm.bin'/>
    <qemu:arg value='-smbios'/>
    <qemu:arg value='file=/usr/share/seabios/smbios_type_0.bin'/>
    <qemu:arg value='-smbios'/>
    <qemu:arg value='file=/usr/share/seabios/smbios_type_1.bin'/>
  </qemu:commandline>

4. Click save
5. Observe everything you just typed disappear
6. Repeat step 3
7. Click save
8. Observe settings correctly integrate