Here is the story in pictures + output .log with "enableNotesLogging" option
1) Going to an existing thought
2) Pasting additional text
3) Going to any another thought and returning back. All pasted text is gone.
I've tested it 4 times. Each time closing the PB program and reloading it. Hence , a 100% problem reproducibility. After attempt 3 I created a blank "output.log" and restarted PB to log only steps 1),2),3) as described above.