Yes! It is related to how StyledStrings.jl renders to HTML. You can see the difference between the header in one side of the test:
<th style = \"text-align: center; font-weight: bold;\"><pre><span style=\"font-weight: 700;color: #803d9b;\">a</span></pre></th>
To the other:
<th style = \"text-align: center; font-weight: bold;\"><span style=\"font-weight: 700; color: #803d9b\">a</span></th>
Notice that in the former we have <pre>.
I remember some discussion about it and having <pre> is just wrong here. Hence, it was probably fixed but not ported to LTS.