Confirmed at https://ask.libreoffice.org/en/question/265713/file-save-as-choosing-a-new-name-and-not-selecting-save-with-password-creates-another-file-protected-password/ . See the comments there.
Copied from there:
I have a calc, ods, file that is protected by a password. Using File -> Save As..., choosing a new name, and not selecting Save with password, does create another file protected password.
If that helps, the following was not published, and not confirmed, at https://ask.libreoffice.org :
1. Before trying to save the copy in an ods format, I tried to save the copy in an 1997-2003 xls format. That xls copy was also saved with a password protection, even though I haven't asked for that. And the password for the xls copy was not the password for the ods file. I didn't try to find out what password it was saved with. 
2. The problem is not only with calc. It could also with writer.