Comment 1 for bug 1590379

I don't think it's a bug, it's normal, they publish examples of non supported ISO but the list isn't exhaustive.

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There's some workaround I read on the internet, but I didn't test, as when an ISO is not supported, I try in another ISO burning software.

1. see if there's no space or unsupported characters in the name of the ISO, or in its path. If there are some, rename using "_" instead of the spaces.

2. You must use a USB 2.0, because USB 1.0 is too slow, and USB 3.0 is not supported.
Source: https://lea-linux.org/documentations/MultiSystem

3. Check the other hardware requirements (on the same "source").
Check if you can burn other ISO.

If 1, 2 and 3 are okay, it's simply a technical limitation, not a bug,
but you can try the workaround given in this page (in French): https://forum.ubuntu-fr.org/viewtopic.php?id=1229491

"-Certaines distributions linux ne sont pas supportées par MultiSystem,
cette méthode va vous permettre d'outrepasser cette limite.
Dans cet exemple nous allons ajouter une version de Debian bien pratique,
car elle intègre tous les firmwares, et en plus elle est multi-architectures."

Tip: Try to use Google translate if you can't read French.