The installation procedure of the official release 7.6.0.3 should replace the installed version 7.5.5.2 and should not be installed parallel to that version. Installation by downloading the gz files and then unzip. Installed by apt deb *.deb.
This is not a bug, the packaged debs allow installing many versions in parallel and only minor versions will replace packages of the same branch (I.e. 7.6.2.1 will replace 7.6.0.3). You can remove an older version with sudo apt remove libreoffice7.5* (And possibly also a "sudo apt purge" step too for a full uninstall). If you want about branches to replace each other, you might be better off using a PPA.