I'm just wondering what service provider you are hosting your contacts with (i.e. iCloud, Google, Microsoft, Yahoo, etc.)?
Around a year ago I too was having very odd editing behavior the Contacts app on Mac. At the time, my Mac was on macOS Sequoia 15.?. After some testing, I discovered that the issues with the Contacts.app user interface seemed to only occur with my Google-hosted contacts. For me, I was ready to move away from Google anyways so I spent the necessary time to move all of my contacts to iCloud instead and then disable Google as a contacts host. Once I did that, with my contacts then stored in iCloud, the Contacts app behaved flawlessly and has continued to do so through now on macOS 26.3. Over the past year I have spent time cleaning up decades of contacts data so I have edited, moved, organized, deleted, and created a lot of iCloud-hosted contacts within the Contacts app on two different Macs and on iPhone. I have not had a single problem since moving my data to iCloud and disabling my Google contacts.
If you are already using iCloud as the only host for your contacts, you may want to use iCloud on the web to work with and clean up your contacts until you can determine what is causing the issue you are experiencing with the Contacts app.