It should be noted there's a difference between "keep" as applied by the BLUE button vs. "keep" via the Series Link "Keep until I delete" (KUID) option. Unfortunately, they're both flagged the same way in the PLAYLIST, even though they are treated differently under certain circumstances.
Let's say you have a series link for CSI set up to KUID with the default "keep 5" episode setting. With this setting, as expected, a CSI episode will never be deleted to make room for a new recording in a "disk space low" situation. A recorded episode of CSI will be deleted, however, to make room for a 6th episode of CSI that needs to be recorded. So in that situation, KUID is really "Keep until the maximum # of episodes has been reached".
Now lets say you have that same CSI SL set-up, but this time it's at the default "Keep until disk full", again with a maximum # of episodes set to 5. If you manually tag one of the recordings on the playlist with the blue button, that episode will truly be a "Keep until I delete" episode. When the time comes to record a 6th episode, that episode will stay, and the number of CSI episodes on the playlist will grow to 6, instead of 5.
As a result, what I do is never use KUID as part of a SL. If I want to be sure a show won't be deleted, I manually "keep" it by tagging it on the PLAYLIST, using the BLUE button.