I feel like this is a magical side effect? Would it be better to throw an error saying that the name is irrelevant unless you specify --format=umd?

The worst thing that happens is that you get extra umd build which means slightly longer build time. Is it that bad side effect all considered?

Throwing error feels like an annoyance to a user with no usefulness. He kinda has to double confirm that he really wants that UMD format :)

Besides, currently, the esm,cjs is build by default. In case you need umd, you have to list all 3 formats there for no good reason. So utilizing the name is kinda escape hatch to "over configure" :)

I can tweak documentation to mention that behavior if you like. It was just an idea so I did not spend too much time with it.

Closing. I just think this is too magical for my taste.