type predicate not working when imported via deno.json file

[johndeighan]

$ deno --version
deno 2.7.11 (stable, release, x86_64-pc-windows-msvc)
v8 14.7.173.7-rusty
typescript 5.9.2

I have a type predicate named isString(), defined in file src/lib/datatypes2.ts as:

export const isString = (x: unknown): x is string => {

	return (typeof x === 'string') || (x instanceof String)
}

Here is my script (in file src/temp/temp.ts:

import {yellow} from '@std/fmt/colors'
import {isString} from 'datatypes2'
// import {isString} from '../lib/datatypes2.lib.ts'

// ----------------------------------------------------

const trace = (x: unknown): void => {

		if (isString(x)) {
			console.log(yellow(x))
		}
	return
}

This version of the script fails when I run deno check src/temp/temp.ts, with the error:
TS2345 [ERROR]: Argument of type 'unknown' is not assignable to parameter of type 'string'.
console.log(yellow(x))
However, if I import the function isString directly from the file, i.e. switch the comment marker // from the third line to the second line, there is no error.
(FYI, my current directory is at c:/Users/johnd/mapper)

Here is my deno.json file (I think only the value for imports.datatypes2 is relevant):

{
	"tasks": {
		"dev": "deno run --watch main.ts"
		},
	"imports": {
		"@std/assert": "jsr:@std/assert@1",
		"@std/fmt/colors": "jsr:@std/fmt/colors",
		"@std/path": "jsr:@std/path",
		"@std/fmt/printf": "jsr:@std/fmt/printf",
		"npm-uglify-js": "npm:uglify-js",

		"base-utils": "c:/Users/johnd/mapper/src/lib/base-utils.lib.ts",
		"croak": "c:/Users/johnd/mapper/src/lib/croak.lib.ts",
		"datatypes": "c:/Users/johnd/mapper/src/lib/datatypes.lib.ts"
		"datatypes2": "c:/Users/johnd/mapper/src/lib/datatypes2.lib.ts"
		}
	}

Things I've tried:

1. I prefer to define functions as constants, using lambda notation, but I've also tried this version with the same results:

import {yellow} from '@std/fmt/colors'
// import {isString} from 'datatypes2'
import {isString} from '../lib/datatypes2.lib.ts'

// ----------------------------------------------------

function trace(x: unknown): void {

		if (isString(x)) {
			console.log(yellow(x))
		}
	return
}

I've also used the zed editor to hover over the value x in the expression console.log(yellow(x)) and depending on which way I import the function, it will show the type of x to be either unknown or string. I assume the same would happen with vscode.