Why do the RMS numbers next to my audiobook chapters look lower after updating to 1.9.13, although I did not re-render?

Because the app now reports the whole-file RMS, the measure ACX uses, instead of the speech-only measure shown before. Nothing in your existing files changed; the number simply describes them the way ACX will judge them. If a chapter shows a value below -23 dB, re-render it (or Start fresh and Render) and it will be mastered inside the range.