The .Plain page variable, is supposed to return the Page content stripped of HTML tags and presented as a string. I detected one case in which it does not correctly:

I have a heading render hook with the following content:

<h{{ .Level }} id="{{ .Anchor | safeURL }}">
  {{ .Text | safeHTML }}
  {{ if .Level | eq 2 }}
  <button type="button" class="anchor tooltip"
    onmouseover="this.dataset.description = 'Copy link to this section'"
    onclick="navigator.clipboard.writeText(location.href.replace(location.hash, '') + '#{{ .Anchor | safeURL }}').then(() => this.dataset.description = 'Copied to clipboard')">
    {{ partial "svg/link.svg" . }}
  </button>
  {{ end }}
</h{{ .Level }}>

When I write in a MD:

## Overview

Blah blah.

Then .Plain returns the following string: Overview this.dataset.description = 'Copied to clipboard')\" Blah blah.

What version of Hugo are you using (hugo version)?

$ hugo version
hugo v0.99.1-d524067382e60ce2a2248c3133a1b3af206b6ef1+extended darwin/arm64 BuildDate=2022-05-18T11:18:14Z VendorInfo=gohugoio

Does this issue reproduce with the latest release?

Yes.