Why does applying 'com.android.library' plugin break references?

I have a build.gradle file for my library that contains apply plugin: 'com.android.library'.

In my MainActivity.kt file I have:

package com.foo.networkdiagnostics

import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import androidx.appcompat.widget.AppCompatButton
import androidx.appcompat.widget.AppCompatEditText
import androidx.appcompat.widget.AppCompatTextView

class MainActivity : AppCompatActivity() {
    lateinit var text: AppCompatEditText
    lateinit var result: AppCompatTextView

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        text = findViewById(R.id.sample_text)
        result = findViewById(R.id.result)

When I have that apply plugin: 'com.android.library' in my library's build.gradle file, I start seeing errors like this in my MainActivity.kt:

Unresolved reference: layout :15
Unresolved reference: id :17
Unresolved reference: id :18
Unresolved reference: id :20

If I comment out that apply plugin line, the "Unresolved reference" errors go away. What is causing this to happen?

This is Android Studio Arctic Fox | 2020.3.1 Patch 3

from Why does applying 'com.android.library' plugin break references?