esp32+sim800c接收到1包数据后就断开了，为什么？

环境是ESP32+SIM800C的硬件，运行的是Pppos client_main的例子。发现每次接收到MQTT下发的命令后，modem就断开了。如下的log:
[0;32mI (1141) pppos_example: GOT ip event!!![0m
[0;32mI (1151) pppos_example: PPP state changed event 266[0m
[0;32mI (1161) pppos_example: MQTT other event id: 7[0m

【2021-11-10 13:45:41:738】[0;32mI (2451) pppos_example: MQTT_EVENT_CONNECTED[0m
[0;32mI (2461) pppos_example: sent subscribe successful, msg_id=58484[0m

【2021-11-10 13:45:41:843】[0;32mI (2651) pppos_example: MQTT_EVENT_SUBSCRIBED, msg_id=10995[0m

【2021-11-10 13:45:42:048】[0;32mI (2811) pppos_example: MQTT_EVENT_SUBSCRIBED, msg_id=26615[0m
[0;32mI (2851) pppos_example: MQTT_EVENT_SUBSCRIBED, msg_id=58484[0m

【2021-11-10 13:45:54:633】[0;32mI (15351) pppos_example: MQTT_EVENT_DATA[0m
TOPIC=v1/dn/cmd

DATA={"taskId":15,"serviceIdentifier":"open_cmd","productId":15098425,"payload":{"open_request":6},"deviceId":"15098425TMB56E55443388"}


【2021-11-10 13:45:55:355】[0;31mE (16071) esp-modem: esp_dte_handle_line(114): handle line failed[0m
[0;33mW (16071) pppos_example: Unknow line received: ~!EH[0m

【2021-11-10 13:45:57:261】[0;32mI (17981) pppos_example: Modem PPP Stopped[0m
[0;32mI (17981) pppos_example: PPP state changed event 267[0m
[0;32mI (17981) pppos_example: PPP state changed event 265[0m
[0;32mI (17981) pppos_example: PPP state changed event 262[0m