APT CURL etc. expect lower case HTTP_PROXY variables

We have 4 (presumably) identical Ubuntu 20.04 machines and one of them expects the environment variables HTTP_PROXY, HTTPS_PROXY AND NO_PROXY to be lower case.

Programs such as curl and apt work perfectly on all servers but the one. When we switch HTTP_PROXY to http_proxy etc that strange server is happy. Before that apt and curl don't see the proxy.

None of the other servers have the lower case versions of these variables.

• What could be the reason that one server expects lower case?
• What could be the reason that all other servers work with lower case?