AWS CDK Not Finding Custom .d.ts files in project on Synth

I have a TypeScript CDK Project. Per the recommendation I read somewhere, I've put both my infrastructure and runtime code into the same project.

The runtime code is required to use an obscure js library which does not have any type definitions. I've created a custom .d.ts file with types as described here. The TypeScript compiler is happy with this and tsc builds correctly.

When however I run cdk synth I get the "Could not find a declaration file for module 'xyz'" error. I'm guessing that the CDK can't find the definition file, however it's in the same location as other .ts files.

I've edited cdk.json and removed "**/*.d.ts" from the exclude filter to no effect.

tsconfig.json:

    {
      "compilerOptions": {
        "target": "ES2020",
        "module": "commonjs",
        "lib": [
          "es2020"
        ],
        "declaration": true,
        "strict": true,
        "noImplicitAny": true,
        "strictNullChecks": true,
        "noImplicitThis": true,
        "alwaysStrict": true,
        "noUnusedLocals": false,
        "noUnusedParameters": false,
        "noImplicitReturns": true,
        "noFallthroughCasesInSwitch": false,
        "inlineSources": true,
        "experimentalDecorators": true,
        "strictPropertyInitialization": false,
        "typeRoots": [
          "./types",
          "./node_modules/@types"
        ],
        "outDir": "./out",                                   
        "esModuleInterop": true,                             
        "forceConsistentCasingInFileNames": true,            
        "sourceMap": true,                                
      },
      "include":[
        "lib",
        "bin",
        "lib/**/*",
        "bin/**/*",
        "types"
      ],
      "exclude": [
        "node_modules",
        "cdk.out",
        "./out/**/*",
        "test/**/*.spec.ts"
      ]
    }

cdk.json:

    {
      "app": "npx ts-node --prefer-ts-exts bin/LeopardGeotabApp.ts",
      "watch": {
        "include": [
          "**",
          "lib/geotab/mg-api.js.d.ts"
        ],
        "exclude": [
          "README.md",
          "cdk*.json",
          "**/*.d.ts",
          "**/*.js",
          "tsconfig.json",
          "package*.json",
          "yarn.lock",
          "node_modules",
          "test"
        ]
      },
      "build": "",
      "versionReporting": true,
      "context": {
        "@aws-cdk/aws-lambda:recognizeLayerVersion": true,
        "@aws-cdk/core:checkSecretUsage": true,
        "@aws-cdk/core:target-partitions": [
          "aws",
          "aws-cn"
        ],
        "@aws-cdk-containers/ecs-service-extensions:enableDefaultLogDriver": true,
        "@aws-cdk/aws-ec2:uniqueImdsv2TemplateName": true,
        "@aws-cdk/aws-ecs:arnFormatIncludesClusterName": true,
        "@aws-cdk/aws-iam:minimizePolicies": true,
        "@aws-cdk/core:validateSnapshotRemovalPolicy": true,
        "@aws-cdk/aws-codepipeline:crossAccountKeyAliasStackSafeResourceName": true,
        "@aws-cdk/aws-s3:createDefaultLoggingPolicy": true,
        "@aws-cdk/aws-sns-subscriptions:restrictSqsDescryption": true,
        "@aws-cdk/aws-apigateway:disableCloudWatchRole": true,
        "@aws-cdk/core:enablePartitionLiterals": true,
        "@aws-cdk/aws-events:eventsTargetQueueSameAccount": true,
        "@aws-cdk/aws-iam:standardizedServicePrincipals": true,
        "@aws-cdk/aws-ecs:disableExplicitDeploymentControllerForCircuitBreaker": true
      }
    }

I'm just executing cdk synth with no parameters.

I got around this by modifying cdk.json

{
  "app": "cp ./types/mg-api-js.d.ts ./node_modules/@types/ && npx ts-node --prefer-ts-exts bin/LeopardGeotabApp.ts && rm ./node_modules/@types/mg-api-js.d.ts",
....
}

If I didn't delete the file after the ts-node step, then if I ran tsc on its own it would fail because of the duplication of the module in the .d.ts file.