TLS ciphersuite without mode of operation
It is a security issue in the sense that it likely shows that somebody doesn't know what he is doing.

TLS_ECDHE_RSA_WITH_AES_256_SHA is not an officially supported cipher suite name (internet search results too quickly degenerate for that). It probably is present in misconfigured OpenSSL servers. I presume it will default to CBC for the actual security. It is extremely unlikely to use ECB mode as that has never been a supported mode, and it would need to somehow ignore the IV if it was to be used.

Just make it the only possible cipher suite on a test server, then connect to it with a client. You'll probably find out that it is actually the CBC ciphersuite TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA with ID {0xC0, 0x14}. The ID is what is actually communicated during the handshake; the name of the ciphersuite is just what you find in the configuration parameters of the server.

It is not possible to just make up a name for a ciphersuite: it needs to resolve to an ID to be communicated within the handshake. In developers language: the ciphersuite name is an enumeration. Unfortunately OpenSSL has the nasty habit of using their own names and patterns for allowing / disallowing enumeration values.

I'd prefer a TLS 1.2 specific GCM ciphersuite and SHA-256 or SHA-512 over that particular ciphersuite.