macOS's OOM manager suspends and pages out processes when it hits a hard OOM event, and then usually (though not always) brings up a window to help you decide what to kill. However, once you do that, you have to then resume those processes that were suspended using that OOM window. If you don't do that (or if the process is terminated for you without the OOM window showing up, then a restart is the simplest solution to restart those processes.