I've found the cause of the issue: according to the documentation, on RedHat, like distros net-tools are not required and not even recommended while they are. OCS inventory Agent should be able to collect data using "ip addr" instead of "ifconfig" but it's not. Once installed net-tools, which contains ifconfig and other stuff, it works. On ubuntu 18.04 and debian 9 the situation is the same (net-tools no longer included in default/minimal installation) but it is a recommended package. I'd suggest you to check if it's installed and, if not, try to install them.
Let me know if this apply even to your situation.