vnet check-ip-address weird result

**Describe the bug**

issuing the command

`az vnet check-ip-address --resource-group ${myrg} --name ${myVnet} --ip-address ${myIpAddress}`

for the private ip of a sql managed instance, gives me the result for an unused/available ip

```
{
"available": true,
"availableIpAddresses": null,
"isPlatformReserved": false
}
```

but I know that the ip address belongs to the SQL managed instance

**To Reproduce**
issue the command :

`az vnet check-ip-address --resource-group ${myrg} --name ${myVnet} --ip-address ${SQLManagedInstanceIP}`

**Expected behavior**
the result should show that the ip is used, something like :

```
{
"available": false,
"availableIpAddresses": [],
"inUseWithResource": "/subscriptions/3g6e9ea-11111-4602-mc76-93e165f0e7cb/resourceGroups/pippo/providers/Microsoft.Network/networkInterfaces/sqlmi01",
"isPlatformReserved": false
}
```

**Environment summary**
az cli installed from apt repository on Linux Mint Tricia, shell used : bash

output of az --version
```
azure-cli 2.5.0
command-modules-nspkg 2.0.3
core 2.5.0
nspkg 3.0.4
telemetry 1.0.4

```

**Additional context**