I agree, the search results should default to sort by new. Regardless of what your default user setting is. This is how it works on https://old.lemmy.world
looks like the default sort for the user history api changed in whatever version of lemmy reddthat is running. Fix in the latest mlmym release 0.0.44 accounts for that now!
Do you have any examples of the user history being jumbled up? Might be caused by the way the lemmy api returns user activity. If the page size is 10 results, it's going to return 5 comments and 5 posts regardless of how recent they are.
Do you have an example you can link to?