Sorry for the late reply.
These play buttons will be corresponding to some metadata of the nodes from RM21.11 (v 1.12) end of November on. You have to add a "nodeClass" with the value "Method". See example create node in the SDK and add the lowest line:
...
// Create metadata
comm::datalayer::MetadataBuilder metadata(builder);
metadata.add_description(emptyString);
metadata.add_descriptionUrl(emptyString);
metadata.add_operations(operations);
metadata.add_nodeClass(Method)
...
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