Try a different brand usb. Different motherboards sometimes don’t support some usb brands. In fact, a Lenovo server I rebuilt refused to boot off certain usbs.
Some motherboards don’t initialise boot off some usb ports. Sometimes the additional ports are on another controller and initialise too slow.
Just try a straight working Ubuntu live boot usb to remove any ventoy from equation. Ubuntu has real signed uefi (and no shim) granted by Microsoft. I think that’s how it works, uefi is a mess.
Try to start isolating all the different factors, and there could be more. It doesn’t necessarily mean anything definitive if it works on another machine.