In the past, Koha remembered which editor style was opened/used last. This behavior appears to have been changed in 23.05. If the Advanced Editor is enabled, it opens by default every time. Previously a new session always started with the Basic Editor by default. There also now appears to be a redirection problem and save failure. To replicate 1. Ensure the Advanced Editor is enabled. 2. Find a record and Edit. 3. Switch to the basic editor. 4. Edit the record by adding the word Test to the title. 5. Click on Save. See that the Advanced Editor opens and the edit is lost.
Orange County Law reports that they're experiencing this issue through the biblonumber edit feature available in reports. The cataloger reports that it is not consistent but consistent enough to be problematic.
I cannot recreate this on master.
It looks like this behavior was caused by custom javascript on my test site. Removing that code solved the problem. Closing this bug report.