Hello Wouter,
this is weird and basically impossible to explain as you said ??
Never seen something like that, I guess is about some plugin.
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<div class="nav-collapse collapse tc-hover-menu-wrapper"><span xmlns:ib="https://rdf.data-vocabulary.org/#"><div class="menu-menu-2-container"><ul id="menu-menu-4" class="nav tc-hover-menu"><li class="ib-crumb ib-added menu-item menu-item-home"><span typeof="ib:Breadcrumb"><a href="https://www.sluisoptiek.nl" rel="ib:url" property="ib:title">Sluis Optiek en Optometrie Putten</a></span></li><li class="ib-crumb ib-added menu-item ib-lastcrumb current-menu-item menu-item-type-post_type menu-item-object-page"><a href="https://www.sluisoptiek.nl/monturen">Monturen</a></li>
[…]
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All this part:
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<span xmlns:ib="https://rdf.data-vocabulary.org/#">
and
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class="ib-crumb ib-added...
<span typeof="ib:Breadcrumb"><a href="https://www.sluisoptiek.nl" rel="ib:url" property="ib:title">Sluis Optiek en Optometrie Putten</a></span>
class="ib-crumb ib-added menu-item ib-lastcrumb
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doesn’t come from the theme