Probably. Now, the error suggests the problem is that the thread classloader of the Solr indexer does not see some classes used by the mediawiki syntax parser. This feels more like a XWikiDocument#parseContent bug or a solr indexer bug than a mediawiki syntax parser one: both should make sure the right wiki (the parsed document’s wiki) is in the context and the current Thread classloader is the context classloader.
You can probably workaround it by making sure mediawiki syntax extension is installed “On Farm” if that’s not already the case.
But I don’t think this have anything to do with the upgrade.