I find myself often winging it with “themself/themselves” and it seems to be like themselves is always colloquially correct when there are multiple preceding nouns you’re referring to…
Otherwise if there’s only one antecedent or whatever, its themself
Be gentle haha
Acc. to Merriam-Webster it’s the opposite - “proved” used to be 4x more frequent than “proven” 50~60y ago, but nowadays they’re equally common in all contexts.
In a more recent context, however, it’s possible that this tendency is being reverted because “proven” is being treated as a participle (even if proscribed), and the distinction between participle and simple past is slowly going away. So it’s possible that you’re noticing a small part of a bigger shift here. (I’m just conjecturing though, this might be wrong; take it with a grain of salt.)