Hi @wolverine,
i did following steps to repro the scenario can you confirm it :
1)created Vnet with 2 subnet
i used this sample as as , click deploy to Azure
https://video2.skills-academy.com/en-us/samples/azure/azure-quickstart-templates/vnet-two-subnets/
after create check the subnet
2-Create linux VM inside the Subnet1 as below
3-Create ACI instance with default image inside Subnet 2
Note: you cannot create ACI inside the VM subnet
Also notice the delegation happened to subnet2 after ACI created , check the vnet1-->subnets
4-connect to the ACI instance and try to ssh or nc to the VM private IP
Connection successful using VM private IP inside Subnet1
I hope this help waiting your feedback