Not generating correct names for different output formats

I'm trying out Microbundle with https://github.com/reduxjs/redux-toolkit . For some reason, Microbundle seems to be ignoring the names that I've specified in the various `main/module/exports` fields, and is trying to reuse the the name from `main` for all output files.

Here's what I'm seeing. Currently, I've got `package.json` configured with these file names:

```js
"source": "src/index.ts",
"main": "dist/a.js",
"exports": "./dist/b.modern.js",
"module": "dist/c.module.js",
"unpkg": "dist/d.umd.js",
"types": "dist/typings.d.ts",
```

Microbundle reports this output:

```
Build "toolkit" to dist:
7.22 kB: a.js.gz
6.45 kB: a.js.br
6.06 kB: a.modern.js.gz
5.42 kB: a.modern.js.br
7.14 kB: a.module.js.gz
6.39 kB: a.module.js.br
7.31 kB: a.umd.js.gz
6.53 kB: a.umd.js.br
```

And the output files are:

```
dist/a.js
dist/a.modern.js
dist/a.module.js
dist/a.umd.js
```

Those are clearly not the file names I specified. Is there something obvious I'm missing here?