Hi, again.
¿Did you make any new findings on the issue?
In the topic (of the link below), which doesn’t mention Pose Library at all, it brings 3 propositions to deal with certain issues users were having with non-saving Key Frames on the Timeline. I don’t know anything about “Keying Set” yet, but it might be limiting the saving of Key Frames perhaps even to the Action Editor (regarding Pose Library use); I’ve just found my file had (as presented in the topic) a non-blank Active Keying Set (in “Keying” dropdown menu, on the Timeline) and also an item in Keying Sets Panel (in Scene Properties Tab, on the Properties Editor); when I removed them both, I was clean to use Key Frames again and the Pose Library was still operating and I’ve check it was indeed saving stuff on the Action Editor as well.
As I was attempting to test insertion of Key Frames, I noticed I was having an alert message that was preventing me to insert Key Frames on the Timeline…and that lead me to that topic. Because I apparently had the same problem as yours and had this Keying Set issues, maybe you should also check that. Good luck.