Hello Yuchan
According to the related RFC 9301, an ITR may receive a response directly from the MS/MR or it may receive a response from the ETR. There are different circumstances that dictate what will happen.
In the event that the MS/MR has a matching entry, it will return a LISP Map-Reply with the known mapping directly. If the MS/MR doesn’t have sufficient information to know if the requested EID exists, then it will forward the request to a device that has more information about the EID being requested. This may be another MS/MR or the actual ETR that has the EID information. The RFC clearly states that in such a situation:
The Map-Resolver does not send any response to the ITR; since the source RLOC is that of the ITR, the ETR or Map-Server that receives the Map-Request over the ALT and responds will do so directly to the ITR.
This makes sense because it is a direct response to the request. Otherwise, if you trace back the response via the MS/MR, you are adding hops to the response, and thus you are delaying it. Does that make sense?
Take a look at the Map-Resolver Processing section of RFC9301 that describes this process in detail.
I hope this has been helpful!
Laz