B757 Captain reported receiving a warning message at brake release to complete the DG acknowledgement procedures. After verifying that DG had been loaded; the acknowledgement message was sent and the proper response was received.

Narrative

Bottom line upfront - aircraft parking brake got released without having acknowledged dangerous goods.While at the gate at ZZZ; the ACARS signal kept going in and out after the AUTO INIT was pushed. The FO (First Officer) was working through the messages while I was doing other preflight duties. It appeared that we were able to receive all required information when the signal came in. We had planned takeoff data; Dispatch release information etc.; but I never saw nor thought to ask the FO if it saw 'no DG (Dangerous Goods)' or not at that time. After completion of all normal checklists to include the Before Push Checklist; when it was time to release the brake; I said to the FO before I released the brake; No DG; right?' He said; no. As soon as I released the brake; we got a warning; "Pilot in command must acknowledge receipt of final message using ACARS misc code prior to aircraft movement.' We were both surprised and the FO apologized; but it was not his fault - and I asked the Tug Driver on the headset if we indeed had DG and they said yes to my continued surprise. The FO and I scanned the messages received page in the ACARS menu and there was no indication received whatsoever on DG. So; we had not received the 'no DG' message; or the message that we had in DG. I then sent the ACARS request for DG and received the final dangerous goods summary. Accordingly; I then sent the code and received the proper response.I thought even if the signal goes out for a time; then when it comes back; messages that were sent but not received were stored in the queue and we would eventually get them. Was/is there something wrong with the system in this case? Should we have still received the 'no DG' if it was sent; or in this case a planned DG message? I looked again; very closely; enroute at our messages received page and there definitely was nothing about DG until I asked for it after getting the warning message."