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
".