Parsoid and old parser parse spaces in interwiki links differently.
Input:
[[meta:foo bar]] [[google:foo bar]]
Parsoid output:
<a rel="mw:WikiLink/Interwiki" href="https://meta.wikimedia.org/wiki/foo%20bar" title="meta:foo bar" class="extiw" id="mwAw">meta:foo bar</a> <a rel="mw:WikiLink/Interwiki" href="//www.google.com/search?q=foo%20bar" title="google:foo bar" class="extiw" id="mwBA">google:foo bar</a>
Old parser output:
<a href="https://meta.wikimedia.org/wiki/foo_bar" class="extiw" title="meta:foo bar">meta:foo bar</a> <a href="//www.google.com/search?q=foo_bar" class="extiw" title="google:foo bar">google:foo bar</a>
Note how Parsoid escaped the spaces using %20, but the old parser escaped them using _ (basically assuming that they are MediaWiki titles).
The old parser's behavior could be considered incorrect, but it might be required for compatiblity (and it is nicer when the links really are MediaWiki titles).
Maybe related: T95473: Interwiki links whose titles contain question marks are not properly escaped in HTML output.