RFC 7540 says below in 5.1. Stream States.
Similarly, an endpoint that receives any frames after receiving a frame with the END_STREAM flag set MUST treat that as a connection error (Section 5.4.1) of type STREAM_CLOSED, unless the frame is permitted as described below.
WINDOW_UPDATE or RST_STREAM frames can be received in this state for a short period after a DATA or HEADERS frame containing an END_STREAM flag is sent.
But ATS dosen't return STREAM_CLOSED when ATS receives DATA, HEADERS and CONTINUATION frames.