Have a look at Comment # 10 of the Rules (support) issue titled "Access and Set Comment extra fields ". It includes a Rule example that looks like so:
{ "rules_test" : { "LABEL" : "Comment bot", "PLUGIN" : "reaction rule", "REQUIRES" : [ "rules" ], "ON" : [ "node_update" ], "IF" : [ { "entity_has_field" : { "entity" : [ "node" ], "field" : "field_status" } } ], "DO" : [ { "entity_create" : { "USING" : { "type" : "comment", "param_subject" : "zzz", "param_node" : [ "node" ], "param_author" : [ "node:author" ], "param_comment_body" : { "value" : { "value" : "This text will be replaced in the next action.", "format" : "filtered_html" } } }, "PROVIDE" : { "entity_created" : { "new_comment" : "New comment" } } } }, { "data_set" : { "data" : [ "new-comment:comment-body:value" ], "value" : [ "node:field-status" ] } } ] } }
The clue to solve your issue (= work around?) is to use that last Rules Action starting with "data_set".