Additional characters appended to id attributes

First off: Thanks for this module, as really seems to work brilliantly!

Problem/Motivation

When generating static pages, some elements get an additional "--2", "--3" or "--4" added to their id attribute (I guess this will increase if the page gets bigger).
I cannot see any scheme where exactly this happens.

Some examples (diff between dynamic and static version):

-    <nav aria-labelledby="block-hauptnavigation-3-menu" id="block-hauptnavigation-3" class="block block-menu navigation menu--main">
+    <nav aria-labelledby="block-hauptnavigation-3--3-menu" id="block-hauptnavigation-3--3" class="block block-menu navigation menu--main focus-within">

-  <h2 class="visually-hidden" id="block-hauptnavigation-3-menu">Hauptnavigation</h2>
+  <h2 class="visually-hidden" id="block-hauptnavigation-3--3-menu">Hauptnavigation</h2>

-    <div data-drupal-messages-fallback="" class="hidden"></div><div id="block-zircon-content" class="block block-system block-system-main-block">
+    <div data-drupal-messages-fallback="" class="hidden"></div><div id="block-zircon-content--3" class="block block-system block-system-main-block">

This breaks CSS styles.

Running on D8.8.8, using Zircon theme (though I think that should be irrelevant).
Static site gets generated via drush.
Tried to clear cache via GUI or drush to no avail.

This happens only on some pages.

Can this be the page itself and tome only triggering it somehow? Any ideas as to where I could look at, then? This problem never seems to happen when using the page dynamically.