esp_modem CMUX truncates response from AT+CSPI?

[ChrisAlfred]

We are using the latest esp_modem and are connected using CMUX mode with PPP on a SIMCOM 7672 (76xx family). Our esp_modem configuration is:

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#
# esp-modem
#
# CONFIG_ESP_MODEM_CMUX_DEFRAGMENT_PAYLOAD is not set
# CONFIG_ESP_MODEM_USE_INFLATABLE_BUFFER_IF_NEEDED is not set
CONFIG_ESP_MODEM_CMUX_DELAY_AFTER_DLCI_SETUP=0
# CONFIG_ESP_MODEM_CMUX_USE_SHORT_PAYLOADS_ONLY is not set
# CONFIG_ESP_MODEM_ADD_CUSTOM_MODULE is not set
CONFIG_ESP_MODEM_C_API_STR_MAX=128
# CONFIG_ESP_MODEM_URC_HANDLER is not set
# CONFIG_ESP_MODEM_PPP_ESCAPE_BEFORE_EXIT is not set
# CONFIG_ESP_MODEM_ADD_DEBUG_LOGS is not set
# end of esp-modem
# end of Component config

When there is little or no PPP activity or not using CMUX, the response to AT+CMUX returned from dce->at() looks like:

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+CPSI: LTE,Online,310-410,0x830B,79778826,148,EUTRAN-BAND2,700,5,5,116,1043,727,16

But when in CMUX mode with PPP activity, we only receive:

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8,EUTRAN-BAND2,700,5,5,116,1043,727,16

Seems to always drop the first 44 bytes.