When compacting a JSON-LD with a @base context, I remarked different behaviour for URIs and URNs:

URIs
With a "@base": "http://example.com", a "@id":"http://example.com/nosec_sc" becomes "@id": "nosec_sc", which is expected.

Playground link

URNs
With a "@base": "urn:something/", a "@id":"urn:something/no_sec" remains the same after compaction though we would expect a "@id":"no_sec".

Playground link