Actually I can confirm that another user is having the same problem as me. https://forums.macrumors.com/threads/activity-monitor-bug-in-macos-ventura.2369342/I think you are using TextEdit. I did say to use BBEdit https://www.barebones.com/products/bbedit/index.html which does understand plists.
All we know so far is that one computer is having this problem.
What is the last modified and last opened date/time on the preference file?
Do these change when you change update frequency in AM?
What the permissions on the file?
Can you see the UpdatePeriod key in the plist file?