esp32s3, esp_modem component, CONFIG_DUAL, AT command error

[lebies]

Hi All,
On the esp_modem component from the IDF Component Registry, and using the PPPoS Client Example, I have an up an running modem, connecting to Internet, all good and well.

Initialized like so:

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ESP_MODEM_DEFAULT_USB_CONFIG_DUAL(0x2C7C, 0x6002, 3, 4)

_intf 3 is AT port, _intf 4 is PPP port.

I send a bunch of AT commands to setup the modem (CFUN, GNSS, etc) before going into DATA MODE, and that works fine.
I then go into DATA MODE with:

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err = esp_modem_set_mode(dce, ESP_MODEM_MODE_DATA);

and that works - I get incoming MQTT messages.

AT some time later, after connecting and getting IP Address, setting up MQTT, syncing time (NTP) - which all works... I issue an AT command to get a GNSS location reading and get the following error:

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E (13:41:32.313) transport_base: poll_read select error 113, errno = Software caused connection abort, fd = 54

and it disconnects my MQTT connection ...

I suspect it has something to do with the AT port being in DATA MODE.

So my question is :
How do I bind a specific interface to a specific mode in dual config, ie intf 4 for PPP and intf 3 for AT? Basically how do I specify which interface type for which modem interface?

Thanks in advance for any help...