Prior to October 12, 2021, it always committed all candidates. After that, it no longer commits in F9/ctrl+enter "run and maintain buffer" operations. This is tested with the lastLineWasRetained method.
The idea is those are temporary experiments and need not clog history until it is complete.
Override this if you don't want all lines added to the history. You can return null to not add it at all, or you can transform it.