GHPullRequestSearchBuilder is built on the previous GitHub API, which does not use advanced search.
On September 4, 2025, advanced search will be enabled by default, but currently, it's not supported (from docs):
Warning
Notice: Search for issues and pull requests will be overridden by advanced search on September 4, 2025. You can read more about this change on the GitHub blog.
I think the github client should support that - as an opt-in now, but after September 4, it will be the only possible way.