1. Open a CSV file 2. Of course, only one sheet will be opened, since CSV files can only contain one sheet by definition 3. Create new sheet 4. Write something in that new sheet 5. Save the file Without any warnings and any errors, only the last sheet is saved, and all the other sheets are lost forever.
What do you expect? https://help.libreoffice.org/latest/en-US/text/scalc/guide/csv_files.html?DbPAR=CALC#bm_id892361
A warning saying that we are going to lose all the other sheets
*** This bug has been marked as a duplicate of bug 92984 ***