The problem is that pnpm uses a strict link structure with the .pnpm directory, and Prettier has problems resolving plugins like this. But when you install with npm, it creates a basic flat node_modules structure, which Prettier can navigate more easily.
I can think in two options for you:

Option 1:
Use pnpm's packageExtensions feature

Add this to your package.json:

"pnpm": {
"packageExtensions": {
"prettier": {
"dependencies": {
"@ianvs/prettier-plugin-sort-imports": "*"
}
}
}
}

This tells pnpm that Prettier depends on your import plugin

Or option 2:

Use:

shamefully-hoist=true

Modify your .npmrc file to make pnpm's dependency resolution more compatible with Prettier. This will make pnpm hoist all dependencies to the root of node_modules, almost like npm, see if it works!!