You go to print a document. Your printer is temporarily unavailable. The print job hits the spooler and sits in a queue. You get bored and launch a game. The printer becomes available. The print driver has to convert the job to PCL or whatever proprietary format the printer uses. Your game hangs or even worse crashes because it's not really designed to run with background tasks on the same cores.

Who do you blame, especially since the usual diagnostic tools of a PC with a full operating system aren't available?

It's a feature very few people will use with lots of sharp edges that people accept on a PC but would find intolerable on a video game console.

It's not running "most of Windows". In particular, as far as I know (no insider knowledge), it's not running any of the Win32 APIs (USER, etc.). So that rules out any printers which require coordination with their desktop app (which, to be honest, to get full functionality is still a shockingly high number of them). It's highly unlikely the XBox hypervisor implements any of the printing APIs, even UWP because this is a video game console, and not only does almost no one own a networked printer, an even smaller group of essentially no one has a printer, opens documents on their video game console, and wants to print them.

Features aren't free. What Xbox feature would you drop to have an engineer go implement printing APIs on them? Is that the best use of their time?

The explicit point of video game consoles is to be a locked down experience, not a PC. Hardware access is generally used for piracy, which game developers do not like.