You have found that connections from ULM agents to Internet Gateway/s through a proxy server (ULM->Proxy->Gateway->SMP Server) are not working well with latest version of cURL.
Mac computers cannot connect to the SMP Server using Proxy -> Gateway.
ITMS 8.7.x
Known issue.
This issue has been fixed with our ITMS 8.8 Release.
The connection from CEM environment through proxy was fixed. Now connections like: SMA Agent->Proxy->Gateway->SMP Server are working.
ULM Agent now switches to CEM mode successfully when connected to SMP Server through Proxy and Gateway.