Agencies regularly send out letters to requesters saying, "Your request has been pending for so long that we wonder if you still want us to answer it. If you don't reply, we will pretend you never filed this FOIA request." This lets them sneakily evade requests.
It's fine for agencies to ask requesters if they'd like to voluntarily cancel their FOIA requests. What isn't fine is for agencies to cancel the requests without ever hearing from the requester. If the agency gets no response to its inquiry, it should continue processing the FOIA request.