According to DIN specification Table 75 CurrentDemandRes should take 25ms. Looking on the log on the EVSE side (DC charging) it looks like it takes ~250ms to respond to CurrentDemandRequest in DIN-based data exchange. i.e.:

DEBUG 2022-11-06 00:56:20,985 - iso15118.shared.exi_codec (282): EXI-encoded message (ns=urn:din:70121:2012:MsgDef): 809a0219ba2bf5b89f4c5ad0d1401200881e0192d8080323e8109463c2019004464b602000
DEBUG 2022-11-06 00:56:21,161 - iso15118.shared.exi_codec (299): Decoded message (ns=urn:din:70121:2012:MsgDef): {"V2G_Message":{"Header":{"SessionID":"66E8AFD6E27D316B"},"Body":{"CurrentDemandReq":{"DC_EVStatus":{"EVReady":true,"EVErrorCode":"NO_ERROR","EVRESSSOC":72},"EVTargetCurrent":{"Multiplier":-1,"Value":15},"EVMaximumVoltageLimit":{"Multiplier":0,"Value":310},"EVMaximumCurrentLimit":{"Multiplier":0,"Value":125},"EVMaximumPowerLimit":{"Multiplier":1,"Value":3875},"ChargingComplete":false,"RemainingTimeToFullSoC":{"Multiplier":0,"Value":1},"EVTargetVoltage":{"Multiplier":0,"Value":310}}}}}
INFO 2022-11-06 00:56:21,162 - iso15118.shared.comm_session (231): CurrentDemandReq received
INFO 2022-11-06 00:56:21,163 - root (782): Send Charging Command
DEBUG 2022-11-06 00:56:21,167 - iso15118.shared.exi_codec (245): Message to encode (ns=urn:din:70121:2012:MsgDef): {"V2G_Message": {"Header": {"SessionID": "66E8AFD6E27D316B"}, "Body": {"CurrentDemandRes": {"ResponseCode": "OK", "DC_EVSEStatus": {"NotificationMaxDelay": 0, "EVSENotification": "None", "EVSEIsolationStatus": "Valid", "EVSEStatusCode": "EVSE_Ready"}, "EVSEPresentVoltage": {"Value": 2857, "Multiplier": -1, "Unit": "V"}, "EVSEPresentCurrent": {"Value": 34, "Multiplier": -1, "Unit": "A"}, "EVSECurrentLimitAchieved": false, "EVSEVoltageLimitAchieved": false, "EVSEPowerLimitAchieved": false}}}}
DEBUG 2022-11-06 00:56:21,403 - iso15118.shared.exi_codec (256): EXI-encoded message: 809a0219ba2bf5b89f4c5ad0e00040800001028548b01018110000c0
INFO 2022-11-06 00:56:21,404 - iso15118.shared.comm_session (415): Sending CurrentDemandRes
DEBUG 2022-11-06 00:56:21,552 - iso15118.shared.exi_codec (282): EXI-encoded message (ns=urn:din:70121:2012:MsgDef): 809a0219ba2bf5b89f4c5ad0d1401200881e0192d8080323e8109463c2019004464b602000

DEBUG 2022-11-06 00:56:21,736 - iso15118.shared.exi_codec (299): Decoded message (ns=urn:din:70121:2012:MsgDef): {"V2G_Message":{"Header":{"SessionID":"66E8AFD6E27D316B"},"Body":{"CurrentDemandReq":{"DC_EVStatus":{"EVReady":true,"EVErrorCode":"NO_ERROR","EVRESSSOC":72},"EVTargetCurrent":{"Multiplier":-1,"Value":15},"EVMaximumVoltageLimit":{"Multiplier":0,"Value":310},"EVMaximumCurrentLimit":{"Multiplier":0,"Value":125},"EVMaximumPowerLimit":{"Multiplier":1,"Value":3875},"ChargingComplete":false,"RemainingTimeToFullSoC":{"Multiplier":0,"Value":1},"EVTargetVoltage":{"Multiplier":0,"Value":310}}}}}
INFO 2022-11-06 00:56:21,738 - iso15118.shared.comm_session (231): CurrentDemandReq received

Unfortunately, after 5 seconds from the first CurrentDemandReq / Res, the car reports "ReadyToChargeState": false, which causes the charging to stop, even though the charging parameters are met by the power unit during these 5 seconds.
Could the duration of the response be the reason why the EV disconnects?
Is 250ms conversion time okay? It was tested on RPi CM4, clean debian, no additional programs running in the background.