Changing the append attribute at runtime results in the following output:

[standalone@localhost:9990 /] /subsystem=logging/periodic-rotating-file-handler=FILE:write-attribute(name=append,value=false)
{
    "outcome" => "success",
    "response-headers" => {
        "operation-requires-reload" => true,
        "process-state" => "reload-required"
    }
}

A reload operation does not stop a file from being appended to. Either the behavior needs to change or the process-state should be "restart-required".

The issue is likely the file name is checked during boot and hasn't changed therefore the it's not set which would cause the stream to the file to be closed and reopened without the append.