It looks like Google has fixed that ongoing bug with the Google cache 404ing after being switched to mobile-first indexing. The bug has been happening for the past several months and as of last Saturday, it seems Google fixed it.

I tested it myself and for every site I tested it with, it worked without serving a 404 page not found error.

Glenn Gabe first notified me of this:

So you no longer need to do that 404 hack to see your page in the Google cache.

Forum discussion at Twitter.