I was able to exactly reproduce as described. Qubes 4.1 works with Ventoy (popular FOSS Multiboot USB software), Qubes 4.2 final release not. Another user here suspected that Qubes installation is searching for a wrong device label.
Ventoy doesn’t seem to be aware of this issue. Hence it would be helpful, if developers could confirm the underlying error cause, and what the installation wizard is expecting exactly as label, so we might forward it as Ventoy GitHub issue.
Also I’d appreciate any workaround. The only alternative I am seeing now is to prepare an extra USB stick just for Qubes as usual.