After struggling with the exact same issue in PyLucene 10.0.0, I've found the solution via https://github.com/fnp/pylucene/blob/master/test/test_PyLucene.py#L149

You are supposed to use parse(String[] queries, String[] fields, BooleanClause.Occur[] flags, Analyzer analyzer). So you are missing flags field.

E.g.:

fields = ["Post", "Title", "Comments", "Subreddit"]
SHOULD = BooleanClause.Occur.SHOULD
query = MultiFieldQueryParser.parse(query, fields, [SHOULD, SHOULD, SHOULD, SHOULD], analyzer)