EC2 Launch Template doesn't start Spot Instance (but works for on-demand instance)

My EC2 launch template doesn't work when using it to launch a Spot instance. The launch template is set to launch a c5.xlarge instance associated to a pre-existing Elastic Network Interface @ index 0.

When launching a spot instance, I receive the following cryptic message, and the spot request fails:

c5.xlarge, ami-b2b55cd5, Linux/UNIX: A network interface may not specify both a network interface ID and a subnet

First off, how can a network interface specify a network interface id? I believe this error means to say "a spot instance may not specify both a network interface ID and a subnet", but I can't be sure. Secondly, my launch template doesn't specify a subnet directly - it only specifies a network interface ID, which in turn specifies the subnet.

As a troubleshooting step, I've tried launching an on-demand EC2 instance directly using the same launch template, via "Launch Templates -> Actions -> Launch Instance from Template" - when I do this, the EC2 instance launches successfully.

I've been able to reproduce this error consistently for over 9 months now, and am surprised that no one else has brought this up. What gives?

Here is my Spot config:

		"MySpotFleet" : {
	        "Type" : "AWS::EC2::SpotFleet",
	        "Properties" : {
	        	"SpotFleetRequestConfigData" : {
					"AllocationStrategy" : "lowestPrice",
					"IamFleetRole" : {"Fn::GetAtt" : ["MyIAMFleetRole", "Arn"]},
					"InstanceInterruptionBehavior" : "stop",
				    "LaunchTemplateConfigs": [
				        {
				            "LaunchTemplateSpecification": {
				                "LaunchTemplateId": { "Ref" : "MyLaunchTemplate" },
				                "Version": { "Fn::GetAtt" : [ "MyLaunchTemplate", "LatestVersionNumber" ]}
				            }
				        }
				    ],
					"ReplaceUnhealthyInstances" : false,
					"SpotMaxTotalPrice" : "5.01",
					"SpotPrice" : "5.01",
					"TargetCapacity" : 1,
					"TerminateInstancesWithExpiration" : false,
					"Type" : "maintain",
					"ValidFrom" : "2021-01-01T00:00:00Z",
					"ValidUntil" : "2050-12-31T23:59:59Z"
	        	}
        	},
            "DependsOn": [
                "MyLaunchTemplate" 
            ]
	    }

If I replace the above Spot config with this on-demand instance config, it works:

		"MyInstance" : {
	        "Type" : "AWS::EC2::Instance",
	        "Properties" : {
				"LaunchTemplate" : {
	                "LaunchTemplateId": { "Ref" : "MyLaunchTemplate" },
	                "Version": { "Fn::GetAtt" : [ "MyLaunchTemplate", "LatestVersionNumber" ]}
				}
        	},
            "DependsOn": [
                "MyLaunchTemplate" 
            ]
	    }

If it helps, here is my Launch Template:

		"MyLaunchTemplate" : {
	        "Type" : "AWS::EC2::LaunchTemplate",
	        "Properties" : {
				"LaunchTemplateName":"MyLaunchTemplate",
				"LaunchTemplateData":{
					"IamInstanceProfile" : {
						"Arn" : {
							"Fn::GetAtt" : ["MyEC2IAMInstanceProfile", "Arn"]
						}
					},
					"ImageId" : "ami-b2b55cd5",
					"InstanceType": "c5.xlarge",
					"NetworkInterfaces" : [
						{
         					"NetworkInterfaceId" : {"Ref" : "MyENI00"}, 
         					"DeviceIndex" : "0"
     					}
					],
					"InstanceInitiatedShutdownBehavior" : "stop",
					"KeyName" : "my-keypair"
	        }
	    }

And the ENI in question:

        "MyENI00": {
            "Type": "AWS::EC2::NetworkInterface",
            "Properties": {
                "Description" : "MyENI00",
                "GroupSet" : [ 
                	{"Ref" : "MySecurityGroup"} 
            	],
                "PrivateIpAddresses": [ 
					{
						"Primary" : true,
						"PrivateIpAddress" : "172.16.0.100"
					},
					{
						"Primary" : false,
						"PrivateIpAddress" : "172.16.0.101"
					}
                ],
                "SourceDestCheck": false,
                "SubnetId": { "Ref" : "MySubnet" }
            }
        }