The anchor of a link to another page is dropped from the URL unless a query string is also set

Links with an anchor and no query string are very common (xwiki.org if full of those) and I don't remember ever having this problem.

I tried on 17.10.9, and I still cannot reproduce what is described:

[[Main.WebHome||anchor="toto"]]

{{velocity}}
$xwiki.getURL('Main.WebHome', 'view', '', 'HTest')
{{/velocity}}

produces the following HTML

<p><span class="wikilink"><a href="/xwiki/bin/view/Main/#toto"><span class="wikigeneratedlinkcontent">Main</span></a></span></p><p>/xwiki/bin/view/Main/#HTest</p>

I think the problem is more what you used as example. AFAIR XWikiSyntax has a pretty special way to treating page content.