That's fine. There's no requirement to "contribute back". Respect the license terms and don't go demanding anything unless you have a support contract and don't expect that you can get a support contract. It's fine to just use something as long as you also don't harass the maintainer as if they owed you something.

The annoying part is that often without a corporate policy for contributing, you are doing the work anyway because you need XYZ from the software, just that it lives in a private fork that will never get upstreamed as a result of policy.